Reimplement a location picker

[paride]

Signal's location picker relies on the proprietary Play Services and has been removed from Cable. The feature is missing and has to be re-implemented using free libraries, possibly using OpenStreeMap. Useful notes:

• The location code has not been removed, but commented out, adding the Location-TODO string to the comment, so it's easy to find where the relevant code is. The three files SignalMapView.java, SignalPlace.java, LocationSlide.java, have been renamed to .java.nogms.
• The location picker should not rely on OsmAnd (or any other app) being installed.
• The URL has to be shared using standard geo: URIs.
• A linkifier for geo: URIs has to be added (a PR implementing this has been submitted to Signal, but it has been rejected, see below).

Relevant Signal issues and PR:

• Location sharing doesn't work signalapp/Signal-Android#6351
• Use OpenStreetMap as location provider signalapp/Signal-Android#5042
• Linkify geo URIs signalapp/Signal-Android#4790 (Linkify geo URIs)
• Add location sharing to the attachment menu signalapp/Signal-Android#4591
• feature request: implement Google Maps API osmandapp/OsmAnd#3526

[paride]

Keep an eye on https://github.com/mapzen/lost and on https://github.com/osmdroid/osmdroid.