Let navigation bar has control if it is focused

Previously, we let the top fullscreen app window stay in immersive mode
when a non-app window gets the focus. However, in some cases, the
navigation bar source window needs to stay visible, and it will make
itself focusable. This CL lets it has control if it is focused.

Fix: 271373861
Test: Swipe to show transient taskbar and long press on the icon.
      See if taskbar stays visible.
Change-Id: Ifb0fff483b1787db34d3b00a3728dd6cf6fa7bb5