Support following typing focus in window mode [3/n].

There are 3 milestones in this feature.
1. Refactor the callbacks for Accessibility in WindowManagerInternal.
2. Implement this feature in such new architecture.
3. Implement the setting choice in preference page.

This CL is for the 3rd milestone.

We add a preference key "magnification_follow_typing" to access and
update the ACCESSIBILITY_MAGNIFICATION_FOLLOW_TYPING_ENABLED value
which controls its corresponding preference page UI.

Bug: 194668976
Test: make RunSettingsRoboTests ROBOTEST_FILTER=
	MagnificationFollowTypingPreferenceControllerTest
	ToggleScreenMagnificationPreferenceFragmentTest
Change-Id: Ib2602c997a535c1acca6a7ddd7bb2c25921f9280