Ensure that mAllAppWinAnimators is not null.

The method showAllWindowsLocked can be called from the layout
side of the WindowManagerService. Previously the AppWindowAnimator
member mAllAppWinAnimators was created on the animation side. If
showAllWindowsLocked was called before the first animation of the
activity an NPE would occur. This fix creates mAllAppWinAnimators
when the AppWindowAnimator is created and also updates it with
current values before calling showAllWindowsLocked.

Fixes bug 6917136.

Change-Id: If3ace22022b65c6888bbb9d0575efe9a45d86475