Open source variant

[niknetniko]

We could create a build variant with no closed source libraries, i.e.

• No Firebase analytics code, disabling is not enough
• No crash reporting, disabling is not enough
• Remove Google Maps integration, replace by Openstreetmaps or just show a list of locations instead.

This is mainly an experiment to use https://developer.android.com/studio/build/build-variants.

The open source variant would use a different sign key, and perhaps some indication that crashes need to be reported.

If this is successful, we might also publish it on F-Droid.

[niknetniko]

Fixed in #319.