Remove hard dependency on Google APIs

[Nutomic]

As I explained in #8, this would make it easier for others to compile the source code, and would make it possible to publish the app on F-Droid.

One way to do this would be to add a new "free" flavor to gradle , which specifies a different AndroidManifest.xml, and disables all features that rely on Google APIs. This is done by Osmand.

Another option would be to just remove everything that requires Google APIs.

[dennisguse]

I forked MyTracks and removed all Google dependencies as well as doing some rework.
Publishing it on Fdroid is work-in-progress.

https://github.com/dennisguse/opentracks