StumblerService should have all the public accessors synchronized

issue #809: I agree, as we increase the concurrency of the app (currently this interface is not used concurrently), it makes sense to have the public interface to the service (which is represented by StumblerService.java) synchronized.
There's a couple places in the StumblerService where you are still calling `mScanManager.isScanning()`, those should be changed to just `isScanning()` to get the synchronized behavior.

There's also a call to `if !mScanManager.isScanning() { startScanning() }` at line 226.  That should just call `startScanning` as the scan manager internally does it's own check of the `isScanning` flag prior to starting a scan.

`mScanManager.setPassiveMode` and `mScanManager.isPassiveMode()` should also probably be synchronized as the flag may be read in one thread, and set by another thread using an Intent.