chore(magnification): add UserContextChangedNotifier class in AccessibilityController

In AccessibilityController#DisplayMagnifier, we create a new UserContextChangedNotifier inner class to wrap the callback notifying logic. Then we can add
extra flag-based notifying decision logic in the internal UserContextChangedNotifier class, as in the next cl.

Bug: 324949652
Flag: NONE
Test: manually
Change-Id: I3dc723bb91903d72f00069964c0ae6aa8e53d0e3