Move backup tests from servicestests to mockingservicestests

This reduces fragmentation of B&R code and makes it just a little bit less confusing.

Previously all of our tests were in servicestests/ but mockingservicestests/ is a "better" location for new tests (it uses ExtendedMockito which allows mocking static methods). There's no point in keeping two directories so this CL moves everything into the new directory.

Test: atest services/tests/mockingservicestests/src/com/android/server/backup*
Change-Id: I1d64f03693b5923dfc192d6ff3390f0b43ccad93