[Java.Interop] JniType.RegisterNativeMethods() is public

[jonpryor]

Context: dotnet/android#1027
Context: https://jenkins.mono-project.com/job/xamarin-android-pr-builder/2048/

Commit 7d51163 changed the visibility of
JniType.RegisterNativeMethods() from public to internal, in an
effort to reduce the likelihood of "user error" from calling
JNIEnv::RegisterNatives() multiple times, as
JNIEnv::RegisterNatives() invocations are replacements, not
"additive"; calling it more than once is generally Wrong™.

Unfortunately, JniType.RegisterNativeMethods() is part of the
public and shipping API, so we can't remove it, as seen in the
xamarin-android PR build output:

ABI BREAK IN: Java.Interop.dll
<!-- start namespace Java.Interop --> <div>
<h2>Namespace Java.Interop</h2>
<!-- start type JniType --> <div>
<h3>Type Changed: Java.Interop.JniType</h3>
<p>Removed method:</p>

<pre>
        <span class='removed removed-method breaking' data-is-breaking>public void RegisterNativeMethods (JniNativeMethodRegistration[]);</span>
</pre>

Revert the visibility of JniType.RegisterNativeMethods() to
public, so that we don't break API compatibility.