Fixes #180: Alert window leaked in Call screen

Author: atsakiridis

restcomm.android.client.sdk/src/main/java/org/mobicents/restcomm/android/client/sdk/RCDevice.java

@@ -269,6 +269,8 @@ public void run() {
                 DeviceImpl.GetInstance().Shutdown();
             }
         }
+        // important, otherwise if shutdown and re-initialized the old RCDevice instance will be getting events
+        RCClient.getContext().unregisterReceiver(this);
         state = DeviceState.OFFLINE;
     }