use FDroid's Repo data structure to ease merging this into F-Droid

Ultimately, we hope to merge as much of this into F-Droid as possible, so
start using some of the F-Droid data structures, like this Repo one.
add FDroid's getSharingUri() and port code to use it

Then we have a single method for generating the repo URI with all of the
various query strings.  Kerplapp still needs to do its own stuff to put
together the scheme and host part.
add support for sending repo URI via NFC

If you are on the Kerplapp main screen and you do the NFC bump, and touch
Kerplapp to send, the URI of Kerplapp's repo will be sent via NFC to the
other side.

Because NFC callbacks must be attached to an Activity, and the relevant
Interfaces weren't added until android-14, this includes a hack to launch a
different Activity depending on the platform that the app is currently
running on.  It seems to be the only way...