Remove dependency on platform-system-robolectric

Dependency on platform-system-robolectric is not needed when mockito
is not necessary.
If mockito is needed, add a static dependency on
mockito-robolectric-prebuilt.

Test: cd tests/robotests && mma
Bug: 38456058
Change-Id: I6fa83e1f4cea55845dbb3369eda4065d93e85d35