android: Use a handler to show/remove notification from main UI thread

This avoids races that were previously seen (e.g. when disconnecting
while connecting, which sometimes showed a "Disconnecting..."
notification).

src/frontends/android/app/src/main/java/org/strongswan/android/logic/CharonVpnService.java

@@ -31,6 +31,7 @@ import android.content.pm.PackageManager;
 import android.net.VpnService;
 import android.os.Build;
 import android.os.Bundle;
+import android.os.Handler;
 import android.os.IBinder;
 import android.os.ParcelFileDescriptor;
 import android.security.KeyChain;
@@ -88,6 +89,7 @@ public class CharonVpnService extends VpnService implements Runnable, VpnStateSe
 	private volatile boolean mTerminate;
 	private volatile boolean mIsDisconnecting;
 	private volatile boolean mShowNotification;
+	private Handler mHandler;
 	private VpnStateService mService;
 	private final Object mServiceLock = new Object();
 	private final ServiceConnection mServiceConnection = new ServiceConnection() {
@@ -158,6 +160,9 @@ public class CharonVpnService extends VpnService implements Runnable, VpnStateSe
 		mLogFile = getFilesDir().getAbsolutePath() + File.separator + LOG_FILE;
 		mAppDir = getFilesDir().getAbsolutePath();
 
+		/* handler used to do changes in the main UI thread */
+		mHandler = new Handler();
+
 		mDataSource = new VpnProfileDataSource(this);
 		mDataSource.open();
 		/* use a separate thread as main thread for charon */
@@ -312,19 +317,33 @@ public class CharonVpnService extends VpnService implements Runnable, VpnStateSe
 	 * the system when low on memory.
 	 */
 	private void addNotification()
+	{
+		mHandler.post(new Runnable()
+		{
+			@Override
+			public void run()
 			{
 				mShowNotification = true;
 				startForeground(VPN_STATE_NOTIFICATION_ID, buildNotification(false));
 			}
+		});
+	}
 
 	/**
 	 * Remove the permanent notification.
 	 */
 	private void removeNotification()
+	{
+		mHandler.post(new Runnable()
+		{
+			@Override
+			public void run()
 			{
 				mShowNotification = false;
 				stopForeground(true);
 			}
+		});
+	}
 
 	/**
 	 * Create a notification channel for Android 8+