Stop scanner interval before starting a new one

This ensures there are no orphaned intervals running.

lib/browser/modules/drive-scanner.js

@@ -133,6 +133,10 @@ driveScanner.service('DriveScannerService', function($q, $interval, $timeout) {
       });
     };
 
+    // Make sure any pending interval is cancelled
+    // to avoid potential memory leaks.
+    self.stop();
+
     // Call fn after in the next process tick
     // to be able to capture the first run
     // in unit tests.