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: m RunFrameworksServicesRoboTests
Bug: 38456058
Change-Id: I60ac070125cf157225247561336288dfe97789c6