Exposed canSwitchUsers() and added an API in UserManager for SUW.

UserManager#canSwitchUsers() is now exposed as a SystemApi for SUW use.
It requires either the MANAGE_USERS or INTERACT_ACROSS_USERS permission.

Added UserManager#removeUser(UserHandle) which mirrors
UserManager#removeUser(int). It requires the MANAGE_USERS permission.

Bug: 120457727
Test: atest com.android.server.pm.UserManagerTest#testRemoveUserByHandle
Test: atest com.android.server.pm.UserManagerTest#testRemoveUser
Test: manual (run setup wizard)
Change-Id: Ie7a641da8ada789931b2b116c907a3ca298490f4