MediaPlayer/TimedMetaData: API updates related to timed metadata

- renamed MediaPlayer.*OnTimedMetaData* to *OnTimedMetaDataAvailable*
- added `final` modifier to TimedMetaData class
- added javadocs for TimedMetaData public methods
- renamed private members and cooresponding accessors for TimedMetaData
- reworded existing TimedMetaData javadoc

Bug: 21045118
Change-Id: If0113307fce08083226fc7f64c4fe6f93553d899