fix(#AlwaysOnMagnifier): update state when setting is changed outside the app

Refactor to keep track of all controllers and notify them all when a key
changes, similar to ColorAndMotionFragment.java

Bug: b/265086261
Fix: 267433655

Test: atest packages/apps/Settings/tests/robotests/src/com/android/settings/accessibility/ToggleScreenMagnificationPreferenceFragmentTest.java
Change-Id: Id44d1d4074318aff8c4c4bc6f07d65b3e0ffc561