[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...
Cosmetics