Allow virtual external cameras on default policy

Virtual devices with a default camera policy can create (only) virtual
external cameras that are associated with the default device context.

Bug: 375609768
Test: atest CtsVirtualDevicesCameraTestCases
Test: atest VirtualCameraControllerTest
Flag: android.companion.virtualdevice.flags.external_virtual_cameras
Change-Id: I7fe67ca0b05c278a4c153d338fc89b5f599acda7