audiotrack: JNIThread is created/joined in Open/Close

Fix Start called after a Stop (When audio track changes).

- JNIThread won't kill itself anymore: In case of error, it will wait for the
  Stop cmd.
- Handle AudioTrack exceptions: don't call audiotrack methods if an exception
  occurred.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>