Check for whether view exists before attempting to resume play.

If the view does not exist, this probably means resumePlayback() was
called as a result of an asynchronous callback that returned after
navigating away from the voicemail playback view. In this case, just
return null.

Bug: 23521761
Change-Id: I75e5e997bce463739612d65f61413d06eae82de3