bootanimation: Only try to shutdown the mediaplayer if it was prepared

The bootanimation tries to shutdown the mediaplayer gracefully by waiting
on an asynchronous shutdown event for 5 seconds.

If, however, there is no boot sound, that asynchronous shutdown event will
never happen and the animation will be stopped for 5 seconds before shutting
down.

Visibly, this fixes the issue where the bootanimation would simply stop near
the end for exactly 5 seconds.

Change-Id: I77f5631368c7d9b9fef7941a6278af9c36032044