Replaced System.Exit calls with FinishAffinity and Start Activity calls

Implemented "RestartApp" method defined in NavigationHelper.java.
 This method is used in ExitActivity.java and ContentSettingsFragment.java

app/src/main/java/org/schabi/newpipe/ExitActivity.java

@@ -6,6 +6,8 @@ import android.content.Intent;
 import android.os.Build;
 import android.os.Bundle;
 
+import org.schabi.newpipe.util.NavigationHelper;
+
 /*
  * Copyright (C) Hans-Christoph Steiner 2016 <hans@eds.org>
  * ExitActivity.java is part of NewPipe.
@@ -48,6 +50,6 @@ public class ExitActivity extends Activity {
             finish();
         }
 
-        System.exit(0);
+        NavigationHelper.restartApp(this);
     }
 }

app/src/main/java/org/schabi/newpipe/settings/ContentSettingsFragment.java

@@ -27,6 +27,7 @@ import org.schabi.newpipe.extractor.NewPipe;
 import org.schabi.newpipe.extractor.localization.ContentCountry;
 import org.schabi.newpipe.extractor.localization.Localization;
 import org.schabi.newpipe.streams.io.StoredFileHelper;
+import org.schabi.newpipe.util.NavigationHelper;
 import org.schabi.newpipe.util.ZipHelper;
 
 import java.io.File;
@@ -255,7 +256,7 @@ public class ContentSettingsFragment extends BasePreferenceFragment {
         // save import path only on success; save immediately because app is about to exit
         saveLastImportExportDataUri(true);
         // restart app to properly load db
-        System.exit(0);
+        NavigationHelper.restartApp(requireActivity());
     }
 
     private Uri getImportExportDataUri() {

app/src/main/java/org/schabi/newpipe/util/NavigationHelper.java

@@ -599,4 +599,16 @@ public final class NavigationHelper {
         intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
         context.startActivity(intent);
     }
+
+    /**
+     * Finish this <code>Activity</code> as well as all <code>Activities</code> running below it
+     * and then start <code>MainActivity</code>.
+     *
+     * @param activity the activity to finish
+     */
+    public static void restartApp(final Activity activity) {
+        activity.finishAffinity();
+        final Intent intent = new Intent(activity, MainActivity.class);
+        activity.startActivity(intent);
+    }
 }