avfvideosrc: Don't wait on main thread for permissions request

Recursively invoking the NSMainLoop can cause crashes in applications that don't expect it. Instead of waiting for permission to be granted, move the wait later - until we actually need device permissions when starting the capture session. That moves the wait into the streaming thread instead of the application thread that's setting the pipeline state to READY.

Instead of a manual state change implementation to open and close the device, use the basesrc start/stop methods that are intended for the purpose.