Mock locations

* Adds `setMockMode(boolean)` to enable mock locations.
* Adds `setMockLocation(Location)` to simulate a single location update.
* Adds `setMockTrace(String)` to simulate a series of location updates from GPX file.

Use `adb push filename.gpx /sdcard/lost.gpx` to test mock GPX traces using the sample app.

I don't feel strongly about this but we could constantize the strings here to improve readability