MediaPlayer: setSubtitleAnchor on a HandlerThread

This ensures that we wouldn't run into the error `Can't create handler
inside thread that has not called Looper.prepare` no matter on which
thread we attempt to set the SubtitleController internally.

Bug: 20821869
Change-Id: Ie53d60b58f5382428e7b7a01deccfa516b61b25f