Draft: Preferences activity

Currently the installable broker says there is no active runtime if it finds more than one. We should really allow the user to choose. This code is highly incomplete, anybody who is better at Android application architecture than me is welcome to do a better job, either starting from here or from the main branch. See also another wip https://gitlab.freedesktop.org/ryan.pavlik/draft-openxr-android-broker-impl/-/commits/room