Verify + improve Mobile AR support for MRTK3

[MaxWang-MS]

MRTK3 already has rudimentary support for mobile AR devices (ARCore, ARKit). However, input is largely missing. We'll need to implement touch-screen based interactors for spatial manipulation from mobile devices (and/or evaluate the ones that already exist as part of XRI by default... but they don't seem too hot 😓)