Track hidden state of window in itself vs. all child windows.

It makes the code easier to follow if the hidden state of a window
is tracked in itself vs. all chils windows. Child windows can call
WS.isParentWindowHidden to get the hidden state of their parent window.

Also, moved the performShowLocked method from WindowStateAnimator to
WindowState since most of the fields modified in the method belong to
WindowState.

And, don't forget the test for the change ;)

Bug: 30060889
Change-Id: I5be13b936a2284fd8fe10218b80fe41cc197d1a7