Fix JobSchedulerServiceTest.

JobScheduler expects the system gallery package to be non-null, so we
have to make sure the test context provides a non null value.

Bug: 146453470
Test: atest FrameworksMockingServicesTests:JobSchedulerServiceTest
Change-Id: Ib84567b006a2f57d17d767e0c08fd542576f979d

services/tests/mockingservicestests/src/com/android/server/job/JobSchedulerServiceTest.java

@@ -107,6 +107,7 @@ public class JobSchedulerServiceTest {
                 .when(() -> LocalServices.getService(AppStandbyInternal.class));
         doReturn(mock(UsageStatsManagerInternal.class))
                 .when(() -> LocalServices.getService(UsageStatsManagerInternal.class));
+        when(mContext.getString(anyInt())).thenReturn("some_test_string");
         // Called in BackgroundJobsController constructor.
         doReturn(mock(AppStateTracker.class))
                 .when(() -> LocalServices.getService(AppStateTracker.class));