[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 (i.e. until a device with support for non-2.0 non-5.1
appears).