Correctly unregister receiver in "qr code" swap view.

Previously the receiver was added but never removed. The result
is that once a swap session is cancelled, the receiver still
gets broadcasts.

F-Droid/src/org/fdroid/fdroid/views/swap/WifiQrView.java

@@ -78,16 +78,19 @@ public class WifiQrView extends ScrollView implements SwapWorkflowActivity.Inner
             }
         });
 
-        // TODO: Unregister this receiver properly.
         LocalBroadcastManager.getInstance(getActivity()).registerReceiver(
-                new BroadcastReceiver() {
+                onWifiStateChanged, new IntentFilter(WifiStateChangeService.BROADCAST));
-                    @Override
+    }
-                    public void onReceive(Context context, Intent intent) {
+
-                        setUIFromWifi();
+    /**
-                    }
+     * Remove relevant listeners/receivers/etc so that they do not receive and process events
-                },
+     * when this view is not in use.
-                new IntentFilter(WifiStateChangeService.BROADCAST)
+     */
-        );
+    @Override
+    protected void onDetachedFromWindow() {
+        super.onDetachedFromWindow();
+
+        LocalBroadcastManager.getInstance(getActivity()).unregisterReceiver(onWifiStateChanged);
     }
 
     @Override
@@ -167,4 +170,11 @@ public class WifiQrView extends ScrollView implements SwapWorkflowActivity.Inner
 
     }
 
+    private BroadcastReceiver onWifiStateChanged = new BroadcastReceiver() {
+        @Override
+        public void onReceive(Context context, Intent intent) {
+            setUIFromWifi();
+        }
+    };
+
 }