Eliminate gap between finalize() and destroy()

 Bug: 6569073
 Only nativeDestroy and nativeStopGL need to be called on the UI thread,
 so split up destroyImpl into 3 functions, and only have the native
 destroy be pushed to the UI thread if necessary. Also make the work that
 is delayed be static without references to the finalizing WebView to allow
 it to be fully deleted immediately after finalization.

Change-Id: I4e424051e69df0bc409af95ca3f3d2b9e58a6b75