Added Android app and .gitignore

Here are the app files and a .gitignore for them

It's not much, and there are a lot of Android events to handle, but it's working (not crashing) and this export is tested.

TODO:
- make sure visited locations list is not erased when screen is turned (need to check)
- make sure positions are passed from the app to the webapp
- automatic position sync when UI is brought up from background
- write positions to a file, when screen is turned and phone is shut down
- make buttons take less space