Move MockitoUtils to server om package

This should definitely be in a shared utility module, but
because of transitive dependencies, adding Mockito to test-utils
isn't that simple.

For now, move it to the overlay package, to be cleaned up
in a later CL.

Bug: 146004674

Test: atest com.android.server.om

Change-Id: I585df0da0f44433d13fb32397d13ad2ad7b5d50c