Do not call "onPrepared" unless the media player is prepared.

In setPlaybackView, if the MediaPlayer is not null, onPrepared is
called. However, there can be a race condition where the MediaPlayer is
not null, but also not yet prepared (since preparation is an
asynchronous call).

Bug: 23228984
Change-Id: I1536de9b2c67c0ae796224a717b1047fac0a8da4