Fix hardly triggering A11y shortcut via gesture when zoom in

When the navigation mode is gestural, users can use two fingers
 swipe up from bottom to trigger A11y shortcut. However, it has conflict
 with scrlloing gesture of magnification. We count the navigation bar
 inset into nonmagnificationReigion in such situation. If the UI is
 in immersive mode, the nagivation bar is invsibile, so it won't
 counted.

Bug: 145775653
Test: manual test:
 1. Enable gesture navigation mode.
 2. Zoom in the screen via gesture.
 3. Turn off magnification via gesture.
Change-Id: I388397d9fb3713987beb44a1611270bf0d944c66