Fix getAnimationTargets so that it returns valid animation target

AppTransitionController#getAnimationTargets traverse ancestors of
activity to find animation targets. Previously it can reach
DisplayContent#mWindowContainers which does not inherit
WindowContainer#createRemoteAnimationTarget thus cannot be a remote
animation target.

After this CL, getAnimationTargets stops traversing if it reaches
WindowContainer which cannot be a remote animation target.

Bug: 149716548
Test: None
Change-Id: Ie94ab8ecd35e9dcc28d0b8f7aaeb058e15b0f80b