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

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.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4096>