Offer Mockito under Ravenwood.

Thankfully the source tree already has a prebuilt variant of Mockito
that is well-supported for mocking on the host side, so this change
transparently offers that under the Ravenwood environment.

Bug: 319647875
Test: atest SystemUiRavenTests
Change-Id: Id87c25ec18cee454defe0dfbd609eb4cbeca5a95