[OpenXR] Manage 'free-hand' and 'controller-held' tracking.

On Quest, use the XR_META_simultaneous_hands_and_controllers extension.
This extension 'decouples' the interaction profiles, so that we can have
a hand with no profile (free-hand) and a hand holding a controller.

On the other devices or if simultaneous_hands_and_controllers is disabled
in the json config, the XR_EXT_hand_tracking_data_source extension is
used, so that hands are not displayed when controllers are held.

Note that on Pico (due to StreamVR ?), controller-held detection is not
working, and so hands are always displayed.