Followup to #639: Fix lifespan of CellScanner

Fix for https://github.com/mozilla/MozStumbler/pull/639.

Introduced memory error, as by design both the service and the client are trying to create a cell scanner.
Create cell scanner in MainActivity startscanning, clear it on stopscanning. This is guaranteed to override the creation in the service, which creates the NoWCDMA scanner by default.

A further note on this split. It only makes sense in the context of Fennec, and for someone who wants to use the service on its own (as it is now designed) on SDK level 18, they won't get wcdma scanning, which is dumb. It won't be here for long in this form, please tolerate it for the time being.