chore(#AlwaysOnMagnifier): Support feature flag flips observation

In AlwaysOnMagnificationFeatureFlag, support adding listener to DeviceConfig to
observe the feature flag flips. If the flag value is changed, notify
AccessibilityManagerService to enable/disable the feature.

Bug: 269414499
Test: manually set flag value with adb shell
      atest MagnificationControllerTest
      atest AccessibilityManagerServiceTest
Change-Id: Ie66972ae8a3ac0ebda2b888bf7265ec1360b6a40
(cherry picked from commit 248a0a79)