Notifies updating magnification switch visibility when the magnification shortcut is triggered

When the magnification is off then a user toggles on magnification by the
magnification shortcut (a11y button or triple-tap gesture),
the switch button should be shown immediately.
When the magnification is on then a user toggles off magnification by the
magnification shortcut, the switch button should be removed immediately.

Bug: 170976389
Test: atest FullScreenMagnificationGestureHandlerTest WindowMagnificationGestureHandlerTest MagnificationControllerTest
Change-Id: I28869f0954d0befd5c675e5f22c303d128e8d8d3