Add user position to the map!

Summary:
Add user position to the map:

• I have used a FolderOverlay, which groups together all the stops Markers, so that only them get thrown away at each load, and the user position stays. It would be much better to just load them all on the map, but that probably takes too much memory.... Or well, manually decide which markers to throw away and which markers to create when the user moves the map around (which might be a LOT better for the performance)

• Use app context for the AppLocationManager, so to avoid memory leaks

Test Plan: check if builds correctly

Reviewers: valerio.bozzolan

Reviewed By: valerio.bozzolan

Subscribers: valerio.bozzolan, Libre BusTO

Tags: Libre BusTO

Maniphest Tasks: T401

Differential Revision: https://gitpull.it/D35