[generator] Improve Javadoc remarks on properties

[pjcollins]

Fixes: #905

Property getters and setters will no longer automatically include
copyright info in their generated <remarks> elements. Instead, this
copyright data will be appended to the <remarks> element after
<remarks> merging is complete.

The custom <summary> and <remarks> merging that happens for
properties will now also use Nodes instead of DescendantNodes to avoid
content duplication.

[pjcollins]

Here is an example of problematic doc generation before and after these changes:

Before:

      <Docs>
        <summary>Return the current requested orientation of the activity. -or- Change the desired orientation of this activity.</summary>
        <value>Returns an orientation constant as used in
            <c>ActivityInfo#screenOrientation ActivityInfo.screenOrientation</c>.</value>
        <remarks>
          <para>Property getter documentation:</para>
          <para>
            <format type="text/html">
              <a href="https://developer.android.com/reference/android/app/Activity#getRequestedOrientation()" title="Reference documentation">Java documentation for <code>android.app.Activity.getRequestedOrientation()</code>.</a>
            </format>
          </para>
          <para>
                    Portions of this page are modifications based on work created and shared by the 
                    <format type="text/html"><a href="https://developers.google.com/terms/site-policies" title="Android Open Source Project">Android Open Source Project</a></format>
                     and used according to terms described in the 
                    <format type="text/html"><a href="https://creativecommons.org/licenses/by/2.5/" title="Creative Commons 2.5 Attribution License">Creative Commons 2.5 Attribution License.</a></format></para>
          <para>Property setter documentation:</para>
          <para>
            <format type="text/html">
              <a href="https://developer.android.com/reference/android/app/Activity#setRequestedOrientation(int)" title="Reference documentation">Java documentation for <code>android.app.Activity.setRequestedOrientation(int)</code>.</a>
            </format>
          </para>
          <format type="text/html">
            <a href="https://developer.android.com/reference/android/app/Activity#setRequestedOrientation(int)" title="Reference documentation">Java documentation for <code>android.app.Activity.setRequestedOrientation(int)</code>.</a>
          </format>
          <a href="https://developer.android.com/reference/android/app/Activity#setRequestedOrientation(int)" title="Reference documentation">Java documentation for <code>android.app.Activity.setRequestedOrientation(int)</code>.</a>Java documentation for <code>android.app.Activity.setRequestedOrientation(int)</code>android.app.Activity.setRequestedOrientation(int).<para>
                    Portions of this page are modifications based on work created and shared by the 
                    <format type="text/html"><a href="https://developers.google.com/terms/site-policies" title="Android Open Source Project">Android Open Source Project</a></format>
                     and used according to terms described in the 
                    <format type="text/html"><a href="https://creativecommons.org/licenses/by/2.5/" title="Creative Commons 2.5 Attribution License">Creative Commons 2.5 Attribution License.</a></format></para>
                    Portions of this page are modifications based on work created and shared by the 
                    <format type="text/html"><a href="https://developers.google.com/terms/site-policies" title="Android Open Source Project">Android Open Source Project</a></format><a href="https://developers.google.com/terms/site-policies" title="Android Open Source Project">Android Open Source Project</a>Android Open Source Project
                     and used according to terms described in the 
                    <format type="text/html"><a href="https://creativecommons.org/licenses/by/2.5/" title="Creative Commons 2.5 Attribution License">Creative Commons 2.5 Attribution License.</a></format><a href="https://creativecommons.org/licenses/by/2.5/" title="Creative Commons 2.5 Attribution License">Creative Commons 2.5 Attribution License.</a>Creative Commons 2.5 Attribution License.</remarks>
        <since version="Added in API level 1" />
        <c>ActivityInfo#screenOrientation ActivityInfo.screenOrientation</c>
      </Docs>

After:

<Docs>
        <summary>Return the current requested orientation of the activity. -or- Change the desired orientation of this activity.</summary>
        <value>Returns an orientation constant as used in
            <c>ActivityInfo#screenOrientation ActivityInfo.screenOrientation</c>.</value>
        <remarks>
          <para>Property getter documentation:</para>
          <para>
            <format type="text/html">
              <a href="https://developer.android.com/reference/android/app/Activity#getRequestedOrientation()" title="Reference documentation">Java documentation for <code>android.app.Activity.getRequestedOrientation()</code>.</a>
            </format>
          </para>
          <para>Property setter documentation:</para>
          <para>
            <format type="text/html">
              <a href="https://developer.android.com/reference/android/app/Activity#setRequestedOrientation(int)" title="Reference documentation">Java documentation for <code>android.app.Activity.setRequestedOrientation(int)</code>.</a>
            </format>
          </para>
          <para>
                    Portions of this page are modifications based on work created and shared by the 
                    <format type="text/html"><a href="https://developers.google.com/terms/site-policies" title="Android Open Source Project">Android Open Source Project</a></format>
                     and used according to terms described in the 
                    <format type="text/html"><a href="https://creativecommons.org/licenses/by/2.5/" title="Creative Commons 2.5 Attribution License">Creative Commons 2.5 Attribution License.</a></format></para>
        </remarks>
        <since version="Added in API level 1" />
      </Docs>