audiotrack: add 5.1 and 7.1 support

Works with SPDIF and HDMI audio output. Android will downmix to stereo if the
audio output is stereo.

7.1 support was added since Android 5.0.

5.1 support was added since the beginning but doesn't work on old devices.
That's why we fallback to stereo if AudioTrack fails to init with more than 2
channels.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>