Verify `android.test.mock` under Ravenwood.

Even though developers are encouraged to use Mockito, existing tests
may still be using this old library for their mocking.

We'll circle back later to consider support for components such as
ContentProvider, Service, etc.

Clean up OWNERS.

Bug: 310268946
Test: atest android.test.mock.ravenwood.tests
Change-Id: Ie3f4ccea9cf6719d61cff277a627825868edc992