android: Don't update state fragments when they are not displayed

Besides that updates don't make much sense when the fragments are not
displayed this fixes the following exception:
	java.lang.IllegalStateException: Can not perform this action after
		onSaveInstanceState

src/frontends/android/src/org/strongswan/android/ui/ImcStateFragment.java

@@ -135,13 +135,33 @@ public class ImcStateFragment extends Fragment implements VpnStateListener
 		return view;
 	}
 
+	@Override
+	public void onStart()
+	{
+		super.onStart();
+		if (mService != null)
+		{
+			mService.registerListener(this);
+			updateView();
+		}
+	}
+
+	@Override
+	public void onStop()
+	{
+		super.onStop();
+		if (mService != null)
+		{
+			mService.unregisterListener(this);
+		}
+	}
+
 	@Override
 	public void onDestroy()
 	{
 		super.onDestroy();
 		if (mService != null)
 		{
-			mService.unregisterListener(this);
 			getActivity().getApplicationContext().unbindService(mServiceConnection);
 		}
 	}

src/frontends/android/src/org/strongswan/android/ui/VpnStateFragment.java

@@ -144,6 +144,7 @@ public class VpnStateFragment extends Fragment implements VpnStateListener
 		super.onStart();
 		if (mService != null)
 		{
+			mService.registerListener(this);
 			updateView();
 		}
 	}
@@ -152,6 +153,10 @@ public class VpnStateFragment extends Fragment implements VpnStateListener
 	public void onStop()
 	{
 		super.onStop();
+		if (mService != null)
+		{
+			mService.unregisterListener(this);
+		}
 		hideErrorDialog();
 		hideProgressDialog();
 	}
@@ -162,7 +167,6 @@ public class VpnStateFragment extends Fragment implements VpnStateListener
 		super.onDestroy();
 		if (mService != null)
 		{
-			mService.unregisterListener(this);
 			getActivity().getApplicationContext().unbindService(mServiceConnection);
 		}
 	}