Changing A11yManagerService to respond to user switching from Lifecycle instead of Intents.

This shrinks the window in which AMS holds an incorrect userId to near non-existence, preventing most if not all user-incurred bugs regarding wrong userIds.

Test: atest AccessibilityManagerServiceTest
Flag: com.android.server.accessibility.manager_lifecycle_user_change
Bug: 393626471
Change-Id: I9a8d21a7449da9f27cbf283584e0960d8c79d37e