Puts mPm.onNewUserCreated inside Binder.withCleanCallingIdentity

PackageManagerService.onNewUserCreated eventually calls
ComputerEngine.getInstalledPackages. 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: 228253821
Test: atest CtsMultiUserTestCases:android.multiuser.cts.UserManagerTest#testCreateUser_withNewUserRequest_shouldCreateUserWithCorrectProperties
Change-Id: Ia06b9d5e6501140541e8d520350368df1abe2f15