Fix leaked receiver when call does not connect.

app/src/main/java/org/thoughtcrime/securesms/service/webrtc/WebRtcCallService.java

@@ -138,6 +138,7 @@ public final class WebRtcCallService extends Service implements BluetoothStateMa
     }
 
     unregisterNetworkReceiver();
+    unregisterPowerButtonReceiver();
 
     TelephonyUtil.getManager(this)
                  .listen(hangUpRtcOnDeviceCallAnswered, PhoneStateListener.LISTEN_NONE);