feat(fullscreen magnification): magnification should not work when no sysui connection

For sysui fullscreen magnification border implementation, if the connection is not established, the magnification should not work. So we won't have a magnified but no border situation.
To achieve this, we add flag and connection check in FullScreenMagnificationController#setScaleAndCenter. If the flag on and no connection, it will just return without activating/changing the magnification, so we can block magnification control from both AccessibilityService.setMagnificationConfig and AccessibilityShortcut.
Besides, when disconnected the fullscreen magnification should deactivate for the consistency. So in AccessibilityManagerService if setMagnificationConnection with null object, it will reset the fullscreen magnification.

Bug: 330420781
Flag: ACONFIG always_draw_magnification_fullscreen_border DEVELOPMENT
Test: manually flip the flag
      atest AccessibilityManagerServiceTest
      atest FullScreenMagnificationControllerTest
Change-Id: Iaf3daf8819182678fe85a78ccb5718b2ecd4889e