Remove the completeAudioAvailable API.

Measurements have shown that the audio track in streaming mode
is pretty efficient as is, thanks to dalvik's JNI implementation.

Also, in streaming mode write( ) blocks until all data has been
copied to the mixer, which is the kind of behaviour we want so
that we don't have to explicitly block in our code. Also,
the java AudioTrack is thread-safe with respect to stop( ) calls
during write, which means we can do everything completeAudioAvailable
did, just as efficiently without it.

Change-Id: I70a8a108d94c7260676673979b5ea29f37fa5db4