Adjust services robotest makefile

In preparation for transport-related tests needed to make those fixes so
that Mockito annotations would work.

1. Switched from mockito-target-minus-junit4 to
mockito-robolectric-prebuilt. Otherwise we would get dexcache == null
error (similar to
https://groups.google.com/a/google.com/forum/#!topic/robolectric-users/gV0_59Kj2ME/discussion)

2. Removed unused frameworks-base-testutils because it included the
ill-suited mockito dependency (as per 1).

3. Tidied up by moving test dependencies to actual test target.

Test: make RunFrameworksServicesRoboTests
Change-Id: Idd4f13a9aa0327d74b58361ad02aa3313d30ac99