Dark Theme: Trying to address #454.
Merge pull request #646 from garvankeeley/fix-dark-style

Dark Theme: issue #454.
Fix for https://github.com/mozilla/MozStumbler/pull/639. Create the 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.
Merge pull request #647 from garvankeeley/fix-creation-of-cellscanner

Followup to #639: Fix lifespan of CellScanner