JobScheduler: Fix FlexibilityControllerTest test failures

Mock the JobSchedulerInternal object as well since the tests
need to create JobStatus objects.

Bug: 437416813
Test: atest
FrameworksMockingServicesTests:com.android.server.job.controllers
Flag: EXEMPT test fix
Change-Id: I210548632beb7de9351d16b7f3dd94aad080bdc9