[mediaplayer] implement fuchsia.media.playback

This CL modifies //src/media/playback/mediaplayer so it implements
fuchsia.media.playback rather than fuchsia.mediaplayer.

//src/media/playback/mediaplayer_tmp still implements
fuchsia.mediaplayer for existing clients

TEST: mediaplayer CQ tests and
  $ fx shell present_view \
fuchsia-pkg://fuchsia.com/mediaplayer_test_util#meta/mediaplayer_test_util.cmx \
--loop <file>

Change-Id: I77c028a1e676e91d9ce9da994fff1a27072197da