addressing https://bugzilla.mozilla.org/show_bug.cgi?id=1038843#c42
- StumblerService, mScanManager and mReporter had null pointer risk,
  changed this to move the risk into those classes, and in those classes
check their members for null.
- removed FirefoxStumbleState enum, was handy when starting the service
  without the intent from Firefox, no longer needed.
braced single-line if(debug) statements
Cleaned up declarations based on review. Changed log to use LOG_PREFIX on each LOG_TAG.