Enable transitions in lockscreen media controller

Media controller now fades between different states. The code for
doing this was already there, but this CL enables them and changes
the behavior of transition's OnPreDrawListener to do the right thing.

Also, this CL fixes a bug in ChangeText found while testing this change.

Issue #11083563 ChangeText transition crashes when KEEP transition type used

Change-Id: I5e04c28e1b5faac017b0a4e49734d9faa7fe79cd