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
6f4fb88538