Use app compat classes

app/src/main/java/org/traccar/client/MainActivity.java

@@ -25,7 +25,7 @@ public class MainActivity extends AppCompatActivity {
     protected void onCreate(@Nullable Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         if (savedInstanceState == null) {
-            getFragmentManager().beginTransaction().replace(android.R.id.content, new MainFragment()).commit();
+            getSupportFragmentManager().beginTransaction().replace(android.R.id.content, new MainFragment()).commit();
         }
     }
 

app/src/main/java/org/traccar/client/MainFragment.java

@@ -17,7 +17,6 @@ package org.traccar.client;
 
 import android.Manifest;
 import android.app.AlarmManager;
-import android.app.AlertDialog;
 import android.app.PendingIntent;
 import android.content.ComponentName;
 import android.content.Context;
@@ -28,7 +27,6 @@ import android.content.pm.PackageManager;
 import android.net.Uri;
 import android.os.Build;
 import android.os.Bundle;
-import android.preference.CheckBoxPreference;
 import android.preference.EditTextPreference;
 import android.preference.Preference;
 import android.preference.PreferenceFragment;
@@ -36,6 +34,7 @@ import android.preference.PreferenceManager;
 import android.preference.TwoStatePreference;
 import android.support.annotation.NonNull;
 import android.support.v4.content.ContextCompat;
+import android.support.v7.app.AlertDialog;
 import android.util.Log;
 import android.view.Menu;
 import android.view.MenuInflater;
@@ -43,9 +42,7 @@ import android.view.MenuItem;
 import android.webkit.URLUtil;
 import android.widget.Toast;
 
-import java.util.HashSet;
 import java.util.Random;
-import java.util.Set;
 
 public class MainFragment extends PreferenceFragment implements OnSharedPreferenceChangeListener {