Fix JobSchedulerTest.

ServiceManager wasn't being mocked so the test kept failing with
"java.lang.ClassCastException: android.os.BinderProxy cannot be cast to
com.android.server.compat.PlatformCompat". Adding the mocking fixes the
tests.

Bug: N/A
Test: atest FrameworksMockingServicesTests:JobSchedulerServiceTest
Change-Id: I07148319f8dd4a31a83b43410f9e06e932faa103