Puts dispatchUserAdded inside Binder.withCleanCallingIdentity

dispatchUserAdded eventually calls handleIncomingUser. And it requires
callingUid to have INTERACT_ACROSS_USERS permission.
That was failing some tests for auto.
This CL makes sure that method is called with clean calling identity.

Bug: 232945806
Test: atest CtsMultiUserTestCases:android.multiuser.cts.UserManagerTest#testCreateUser_withNewUserRequest_shouldCreateUserWithCorrectProperties
Change-Id: I2d0984c5166dd650f81a3a72696cece8054da26a