Start NotificationService as a foreground service

This should prevent the notification from remaining after the app has
been killed by Android.

Fixes #1037.

app/src/main/java/com/beemdevelopment/aegis/services/NotificationService.java

@@ -47,8 +47,7 @@ public class NotificationService extends Service {
                 .setOngoing(true)
                 .setContentIntent(pendingIntent);
 
-        NotificationManagerCompat notificationManager = NotificationManagerCompat.from(this);
-        notificationManager.notify(NOTIFICATION_VAULT_UNLOCKED, builder.build());
+        startForeground(NOTIFICATION_VAULT_UNLOCKED, builder.build());
     }
 
     @Override