MediaPlayer cleanup, part one of N

Get rid of a lot of dead code in MediaPlayer and associated classes,
and simplify MediaPlayer now that there's only a single MediaEngine
to worry about.

Note that this is just the beginning: there's clearly lots more to be
done, and some the layers here probably no longer need to exist.

Review URL:

git-svn-id: svn:// bbb929c8-8fbe-4397-9dbb-9b2b20218538
10 files changed