Fix black screen on app transition.

The layer adjustment to an animating window upon completion was masking
the window behind the mWindowAnimationBackgroundSurface, a DimSurface.
The DimSurface was not being hidden because the step was happening too
late. Swapping the order of performAnimationsLocked and
updateWindowsAppsAndRotationAnimationsLocked fixes this ordering issue.

Fixes bug 6185920.

Change-Id: I0ff64c019e821fa3a92505ac6351f2648897e592