Fix thumbnail animation appearing on going home transition.

We were not dilligent enough about clearing old state in AppTransition,
so parts of old overrides would bleed to the new state. In order to
improve the situation, we have now a single clear method, that will
clear all state that should be restricted to a single transition.

Bug: 25630796
Change-Id: Icb402d05aaa2d1bd356d55e43502442d8fd8cd23