Add enter-animation-done callback for system windows

Existing hidden methods allow activities to be notified when their
windows have completed animating in. This change adds that capability
to system windows using a ViewTreeObserver callback since system
windows lack an activity token.

The first subsystem to use this is the UserSwitchingDialog which was
previously using a 250 msec timeout to dismiss the dialog. That
deadline was often missed leaving the user with no dialog on the
screen during the transition.

Fixes bug 16661752.

Change-Id: I70789e0d9c07112f275e76fb82850926305f290d