Add API for virtual camera device policy and lens facing

- DEVICE_POLICY_DEFAULT doesn't allow the creation of any
virtual cameras.
- DEVICE_POLICY_CUSTOM allows the creation of virtual cameras,
but every virtual device can have at most one virtual camera
with LENS_FACING_FRONT and one virtual camera with LENS_FACING_BACK.

Note that this change doesn't yet add device awareness to the camera
framework.

Test: atest VirtualCameraControllerTest
Test: atest CtsVirtualDevicesCameraTestCases
Bug: 310857519
Change-Id: I5a1e7336e682dfcbd09e775f19dc53659cc77d25