[A11y Magnification Keyboard Focus] Add Settings for Magnification keyboard focus following

* Update user settings to add Magnification following keyboard focus,
  defined by ACCESSIBILITY_MAGNIFICATION_FOLLOW_KEYBOARD_ENABLED.
* Add setting to backup.
* Read setting from full screen and window magnification.
* Set user preference for Magnification following keyboard focus to
  false by default.

Flag: android.view.accessibility.request_rectangle_with_source
Bug: 391679180
Bug: 391877896
Bug: 352134480
Test: Manual local testing, MagnificationControllerTest,
AccessibilityManagerServiceTest

Change-Id: Icc546a54929ece84ebf84a8b9a70fd769153ab94