Switch audiotrack from JNI to BridgeKit.

It seems possible for two worker threads run concurrently, and when they
do, they trample on the global JNIEnv.  The AudioTrack.write call still
uses JNI for efficiency.
Revert part of "Switch audiotrack from JNI to BridgeKit."

 * Restore the C file.

 * Revert the opensles.c changes.

 * Remove the AndroidAudioTrack class.  It is moved to BridgeKit.