Reset layout needed at each animation step.

The member variable WindowAnimator.mPendingLayoutChanges was never
being reset to 0. Consequently once it was set it was causing endless
calls to the layout method.

Fixes bug 6208114, 6220403, 6219546.

Fixed NPE in RecentsPanelView.

Change-Id: Ie529b8f31e535543cb5ae0af9447146306b14eeb