Fix SecurityException when scanning for wifis (#581)

* Fix SE when scanning for wifis

On specific android versions wifiManager.getScanResults didn't
return an empty list but instead threw an exception. Check if
permission has been granted and only if so access scan results.

Fixes #569

* Remove unused log import

app/src/main/java/de/tum/in/tumcampusapp/services/ScanResultsAvailableReceiver.java

@@ -69,6 +69,12 @@ public void onReceive(Context context, Intent intent) {
         boolean locationsEnabled = ContextCompat.checkSelfPermission(context, Manifest.permission.ACCESS_FINE_LOCATION) == PackageManager.PERMISSION_GRANTED;
         boolean wifiScansEnabled = Utils.getInternalSettingBool(context, WifiMeasurementManager.WIFI_SCANS_ALLOWED, false);
         boolean nextScanScheduled = false;
+
+        if (!locationsEnabled) {
+            //Stop here as wifi.getScanResults will either return an empty list or throw an exception (on android 6.0.0)
+            return;
+        }
+
         WifiScanHandler wifiScanHandler = WifiScanHandler.getInstance(context);
         List<ScanResult> scan = wifi.getScanResults();
         for (final ScanResult network : scan) {
@@ -82,7 +88,7 @@ public void onReceive(Context context, Intent intent) {
             }
 
             //if user allowed us to store his signal strength, store measurement to the local DB and later sync to remote
-            if (locationsEnabled && wifiScansEnabled) {
+            if (wifiScansEnabled) {
                 storeWifiMeasurement(context, network);
                 nextScanScheduled = true;
             }