PiP: Update accessibility window as menu shows/hides.

When calculcating accessibility regions w/ shell root, currently since
shell root has no information on the child views' alpha, visibility,
etc, it is always detected as active by the a11y controller. Update the
accessibility region for PIP as menu shows and hides so that the
controller can take this into consideration.

Bug: 192524653
Test: Switch access to PIP when menu shows - able to navigate around
menu items. Switch Access to PIP when menu is gone - able to select PIP
itself.

Change-Id: I7d5649ecbf98751a117c58ac8127584bc3196910