Manually discover virtual camera service

The virtual camera service is not declared in the VINTF manifest.
Currently libcameraservice only looks for declared AIDL ICameraProvider services

This CL checks if the virtual camera service is running and adds the
corresponding CameraProvider to the CameraProviderManager

Test: CameraProviderManagerTest.AidlVirtualCameraProviderDiscovered
Test: CameraProviderManagerTest.AidlVirtualCameraProviderDiscoveredOnInit

Bug: b/270352264
Change-Id: I15667ea18bc32c90f76c1e25faa5c42199495aab