Migrates users to the AccessibilityMenu in system.

The migration is performed when A11yManagerService prepares its user
state for a newly-available user.

The following are automatically migrated:
- The enabled state
- The shortcut state

Bug: 261252772
Test: atest AccessibilityManagerServiceTest
Test: Enable A11yMenu-in-AAS,
      Set up various shortcuts for A11yMenu-in-AAS,
      adb sync system_ext to put A11yMenu-in-System on the device,
      reboot and observe A11yMenu-in-System enabled by default.
Change-Id: Id91b68b78a948857949fb6975988dc524e365fad