Activity windows should not be focusable if not attached to the hierarchy

Top focused activity window was set to invisible and was removed from
display. While comparing the current focused window z-order with
other windows (in order to update focused window), an exception was thrown
because the current focused window was already removed from the hierarchy.

Bug: 144146795
Test: existing tests
Change-Id: If50012d5d3cbe88d0644daa8ef035007f48d1e5f