Support following typing foucs in window mode [1/n].

There are 2 milestones in this feature.
1. Refactor the callbacks for Accessibility in WindowManagerInternal.
2. Implement this feature in such new architecture.

This CL is for the 1st milestone.

The goal of refactor:
When the window manager invokes callbacks which are related to
contextual changes affecting the screen magnification feature, it will
be handle by the MagnificationController first now. Then, it will
dispatch the events to the activated magnification controller such as
FullScreenMagnificationController or WindowMagnificationManager.

We use delegate design pattern to make MagnificationController as proxy.
It will assign a specific delegate when the activated controller is changed.
Then, it will dispatch the events from window manager to activated
magnification controller. Therefore, the activated magnification
controller will handle this event.

For now, we introduce WindowManagerEventDispatcher in
AccessibilityControllerInternalImpl to pass the callback events to
MagnificationController. However, AccessibilityControllerInternalImpl
would be refactored since it should have no nested class relation with
AccessibilityController.

Bug: 194668976
Test: atest FullScreenMagnificationControllerTest
	atest WindowMagnificationManagerTest
	atest MagnificationControllerTest
	atest CtsAccessibilityServiceTestCases
Change-Id: Ic4d87c48a15b9179afeda48132635adc8e86107c