VideoView2: Applying missing experimental code for enabling subtitle

This will be repaced once it is integrated with MediaSesssion2 though.
It detects subtitle track and informs to MCV2 to via MediaSession event
so that MCV2 enable CC button.

TimedText related code is added for debugging purpose. It will be
removed later.

Test: VideoViewTest app
Change-Id: I3a8fef04b3cb4875f091f0b72d3a084bf5fa94ac