[android] Add 5.1 PCM playback support

Tested on ADT-1 Android TV device.

This commits add code for supporting arbitrary channel layouts with the
AudioTrack API. However, at least on ADT-1 any non-5.1 layout gets
downmixed to 2.0, so add a compile-time define for limiting playback to
2.0 and 5.1 (so that e.g. 4.0/5.0 content is played back as 5.1), at
least for now.



Do you think this way (code fully supports everything but disabled for now with LIMIT_TO_STEREO_AND_5POINT1) is OK or should I instead have the absolute minimum code for stereo/5.1 only, i.e. without the channel map stuff? I thought it might be nicer this way, as this allows easily testing for non-5.1 in the future by just toggling the define, instead of having to manually re-add the channel map stuff every time...
I don't see it.

If CJNIAudioTrack gets constructed successfully, it will get assigned to jniAt which is returned back to caller which frees it on deinit.

If there is an exception, CJNIAudioTrack is freed and jniAt remains NULL.