Don't let fullscreen app window hide status bar...

... when notification shade can receive keys.

If notification shade is the focused window, we won't let the top
fullscreen app to hide status bar. If there is a focused window on top
of the focusable notification shade, we should keep preventing the top
fullscreen app from controlling status bar.

Fix: 171223838
Test: atest InsetsPolicyTest
Change-Id: I6d61e3936cb273a8f4dd8546334f751f3a86f4b8