Fix AccountManagerServiceTest

Unregister AccountManagerInternal before creating a new instance of
AccountManagerService, otherwise it will cause:
java.lang.IllegalStateException: Overriding service registration

Also overrode MockPm.addOnPermissionsChangeListener to be a noop and
not throw an exception.

Bug: 31071079
Change-Id: I0d24232eba655a68bf3fb38d7aa9edb92d469cea