Add two new hooks to fix broken CentralAuth unit tests

Add two new hooks to fix broken CentralAuth unit tests

* UnitTestsAfterDatabaseSetup
* UnitTestsBeforeDatabaseTeardown
Bug T168802 necessitates that CentralAuth creates its tables
before any user tests are run, due to its extensive hooking
into everything. This change adds two hooks for that, called
after DB initialization is done and before teardown begins. The
corresponding CentralAuth change is
at I618840fafd22d9b6471eb470ef0414e354aa17f5

Bug: T168802
Change-Id: If7050513719833d4167a24283885d7c10a25856b