[A11y Magnification Keyboard Focus] Add toggle setting for Magnification following keyboard focus

* Add toggle setting for Magnification following keyboard focus, hidden
  behind a flag.
* Add to non-indexable keys.
* Add strings for toggling keyboard focus following.
* Add tests for preference.

NO_IFTTT= Updated in both locations. Linter not registering.

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

Change-Id: I05585f06d427b1f6f88099ef304043d0bf664f9c