[Javadoc] Merge return values on bound properties

[pjcollins]

Context: https://developer.android.com/reference/android/animation/Keyframe#setInterpolator(android.animation.TimeInterpolator)

The Java source for Keyframe.setInterpolator incidentally includes a
@return tag, even though the return type is void:

    /**
     * Sets the optional interpolator for this Keyframe. A value of <code>null</code> indicates
     * that there is no interpolation, which is the same as linear interpolation.
     *
     * @return The optional interpolator for this Keyframe.
     */
    public void setInterpolator(TimeInterpolator interpolator) {
        mInterpolator = interpolator;
    }

This causes us to generate multiple <returns> C# doc comment tags on
the Android.Animation.Keyframe.Interpolator property, as the setter
and getter both include the same @return tag:

    /// <summary>Gets the optional interpolator for this Keyframe. -or- Sets the optional interpolator for this Keyframe.</summary>
    /// ...
    /// <returns>The optional interpolator for this Keyframe.</returns>
    /// <returns>The optional interpolator for this Keyframe.</returns>
    public virtual unsafe Android.Animation.ITimeInterpolator? Interpolator {

The mdoc.exe tool does not handle this well, and will continously
append a <value> element on this property every time the tool runs
against our improperly formatted C# doc comments:

    <Docs>
      <summary>Gets the optional interpolator for this Keyframe. -or- Sets the optional interpolator for this Keyframe.</summary>
      <value>The optional interpolator for this Keyframe.</value>
      <value>The optional interpolator for this Keyframe.</value>
      <value>The optional interpolator for this Keyframe.</value>
      ...
      <since version="Added in API level 11" />
    </Docs>

Fix this by attempting to merge returns elements for property getters
and setters.