[Android] Add support for disabling dynamic thread merging.

[0xZOne]

We make JNI functions callable from any thread here, not just the platform thread, to allow disable dynamic thread merging.

// |ExternalViewEmbedder|
bool AndroidExternalViewEmbedder::SupportsDynamicThreadMerging() {
  return false;
}

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[chinmaygarde]

This will put the Android configuration in a mode where the platform and raster runners are always the same. This is a performance pessimization and not the right fix.