Remove mockito and objenesis from robolectric

Remove mockito and objenesis from the classpath during Robolectric
test runs, to avoid confusing the classloader when tests upgrade to
mockito 2.

Added objenesis to the (now deprecated) platform-system-robolectric
target so mockito continues to work on existing tests.

Test: Existing robolectric tests pass
Bug: 38456058
Change-Id: I33031ce10da918d45b1bc0db06886399963e376c