Notify device owner when user is started / stopped / switched

- Similar to onUserAdded / onUserRemoved, notify DPC when user is started / stopped / switched, as these operation is asynchronous.
Use cases:
1. As a asynchronous callback for startUserInBackground / stopUser / logoutUser / switchUser
2. For COMP, can be used to detect work mode turning on or off

Bug: 72092112
Test: com.android.cts.devicepolicy.DeviceOwnerTest#testCreateAndManageUser_StopOnStart
Test: com.android.cts.devicepolicy.DeviceOwnerTest#testCreateAndManageUser_LogoutOnStart
Change-Id: I6fa28711419053694131d7792f3e0d1d2e76ac29