Explicitly disconnect in MediaPlayer::reset

Even without calling disconnect in MediaPlayer::reset,
MediaPlayerService::Client (which lives on the other side of a
binder with MediaPlayer) would still perform a disconnect in its
destructor. Object destruction across binder is not synchronous, but
method calls are. This commit merely makes sure that the disconnect
happens deterministically before reset returns.

Bug: 22451733
Change-Id: Id9146a63a1872d68087a252aa614248b16d2556d