Fix leak with transitions when views get removed

Transitions, when started, add an OnPreDrawListener to the current
ViewTreeObserver (which is global to the view hierarchy). This listener
is removed when the listener is called.

It is possible to add this listener and then remove the view from
the hierarchy before the listener is called. This could result in
either the listener not getting called at all (since there was no
drawing event) or (in the case of this bug) the listener getting called
when the sceneRoot had no AttachInfo (which is the case when that
root has been removed from the hierarchy). This results in the listener
trying to remove itself from a *different* ViewTreeObserver than the one
it added itself to, leaving the actual listener still sitting on a list
of listeners in that original VTO. This can result in a growing list of
listeners and a growing amount of work that gets done on every frame.
It can also lead to a serious memory leak, since the objects referred to
by the transition may be non-trivial (as in the case of this bug).

The fix is to add another mechanism for the listener to get removed.
Specifically, we now listen for detach events on the sceneRoot. If that
view gets detached before the listener is called, then we have a chance to
remove it from the correct VTO before the AttachInfo becomes null.

Issue #11307391 keyguard is slow after updating to krt16c and playing music

Change-Id: I108413ea2f18f5351df0a11d4ae56fec0b4aa154