Replace observer for USER_SWITCHED broadbast by a UserSwitchObserver

Replace a broadcast intent observer for USER_SWITCHED by a
UserSwitchObserver instance registered to the ActivityManager.

Add an early reaction to user switch by implementing the method
onUserSwitching, which is triggered before any app is notified. Reload
user vibration settings based on the new user ID.

Fix: 308932536
Test: atest VibrationSettingsTest
Change-Id: I185903a0679da01409bcc5db6e0703d451bae0c2