audiotrack: refactor Configure and Start

Rename JNIThread_Configure into JNIThread_NewAudioTrack. This function doesn't
touch any vlc format anymore, it does only Android AudioTrack configuration.
The VLC init part is done by the Start function. JNIThread_NewAudioTrack can be
called up to 3 times by Start with different channels and format until if find
a working configuration. Indeed some devices don't support 5.1, or FL32.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>