MediaPlayer.MTP: fix issue when main thread had no looper

We fall back to using the application's main looper similarly to MP,
and even create our own looper if MP was created outside of an
application context.  Always creating our own looper is a waste
of resources.

Also resolved issue with MP.reset() when created outside of an
application context, and removed MP.MTP's unused getHandler().

Change-Id: I9c3b8a2f05a9e231ae9c008bc1a48d6b991cec1d
Signed-off-by: Lajos Molnar <lajos@google.com>
Bug: 10634238