audiotrack: add java module

audiotrack: add java module

This module is based on the old native audiotrack but uses the public Java
AudioTrack API via JNI. All JNI operations are done in a separate thread in
order to avoid the overhead when attaching and detaching the current thread to
the Java VM.

News since native audiotrack:
 - Flush discards audio immediately (if not wait).
 - It can handle a mediaserver crash and restart (for Android 5.0 and after).

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>