Refactors FullScreenMagnificationController#onMagnificationChangedLocked

Uses
MagnificationInfoChangedCallback.onFullScreenMagnificationChanged
callback method to notify magnification hange instead of
accessing the public method of AccessibilityManagerService directly.

It reduces the interaction between AccessibilityManagerService and
FullScreenMagnificationControlller.
So that MagnificationController would take responsible for handling
notifyMagnificationChange().

Bug: 218540156
Test: atest FullScreenMagnificationControllerTest,
   atest FullScreenMagnificationGestureHandlerTest,
   atest MagnificationControllerTest,
Change-Id: Ib9f31681e442f1700a76250a5206199bad458c84