Fix injection of local services in SupervisionService.

Before, we're trying to inject them in the constructor, resulting in a null instance of DevicePolicyManagerInternal in most cases. To solve this, I decided to adopt the Injector pattern that is used in other services.

FLAG: android.app.supervision.flags.enable_sync_with_dpm
Test: atest SupervisionServiceTest
Bug: 358134581
Change-Id: Ibb97665a9f7201941a330e70df7e4cb0f637d9ee