Fix MapLocationListener pausing GPS

@garvankeeley This might fix #1317

How to reproduce the bug:
- start using the app
- at any point: do not move so that the scanning is paused
- **make sure that the app is not visible when you start moving and scanning is resumed**
  - e.g. enabled lockscreen (I suppose this happens a lot), use another app, press home button
- after scanning was resumed: stop scanning, press home button

Result:
- GPS is still active

I think what happens here is this:
- hiding app: `MapFragment.onPause` calls `mMapLocationListener.removeListener`
- moving after pause: `MapFragment.showPausedDueToNoMotionMessage(false)` re-enables GPS
- app becomes visible: `MapFragment.onResume` creates a new `MapLocationListener`, the reference to the old `MapLocationListener` with the active GPS is lost

This PR makes sure that `pauseGpsUpdates` does not have any effect after `removeListener` was called.
When this is null, then any following calls to `pauseGpsUpdates` will not have any effect, so the bug is fixed. But the changes in `MapFragment` alone are sufficient to fix the bug, I just felt like this would be less likely to regress because it does not depend on code outside the class.
I can push the updated version later.