CastPlayer does not support `mediaMetadata`

[marcbaechinger]

Observed behaviour

CastPlayer always returns MediaMetadata.EMPTY for getMediaMetadata(). Accordingly Player.Listener.onMediaMetadatachanged is never called.

Expected behavour

a) getMediaMetadata() returns a MediaMetadata instance that corresponds to the current MediaItem being played (see issue #25 also). b) The callback Player.Listener.onMediaMetadataChanged is called accordingly when the metadata returned by getMediaMetadata() changes.