Send AppTransitionFinish notification when there was no animation

Activity#onEnterAnimationComplete() is the hook that we advise app
developers to use to know when they are allowed to start drawing
(so they don't collide with the window transition animation).
However, it's not invoked if the window transition has no animation
(e.g. by calling Activity#overridePendingTransition(0,0).

Bug: 20823935
Change-Id: I5b286968b0cd3351e9a9224294d0a1e7faf8c654