Expose camera open event to virtual camera owner

Introduce a new callback in VirtualCameraCallback to notify the owner of
the camera being opened by the client appliation.

Bug: 409241122
Test: atest VirtualCameraTest
Flag: android.companion.virtualdevice.flags.virtual_camera_on_open
Change-Id: Ie6656624fc0efe3c83ba9fe34faaa9837f40042f