Add unit tests for RemoteProvisioningRegistration

These tests are checked in separately from the main registration
implementation because:
1. We do not yet have an OWNERS file for our tests
2. A small API change is required first (making RegistrationProxy not
   final so we can mock it)

Test: Ran this test
Bug: 261888917
Change-Id: Ib67b6fb398ad85a260bba07e67ad1860e68a4fb1