Update to mockito-core 2.7.1

Code changes during this update:
 - Replace deprecated MockJUnitRunner with MockitoRule
 - Some tests include unncessary stubs. This CL simply allows them
   to continue by relaxing the strictness of the stub verification
   in those cases.
 - Remove use of mockito internal Sets class with "new HashSet<>"
 - Tests that assumed that anyString() will match nulls will now fail
   since the behavior has changed in mockito 2.0+. Such uses have been
   replaced with nullable(String.class)
 - Replace Matchers w/ ArgumentMatchers

Test: existing tests pass
Change-Id: Ie2b898da1ee61e7a4779919fec929efa47d2e0f9