PreferenceFragment overlaps toolbar

As the title, my PreferenceFragment overlaps my toolbar. I've already tryed different solutions but the problem persists. Hopes in any help. Here is my code.

Activity

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;

public class MyActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_settings);

        Toolbar toolbar = (Toolbar) findViewById(R.id.app_bar);
        setSupportActionBar(toolbar);

        // Display the fragment as the main content
        getFragmentManager().beginTransaction().replace(android.R.id.content, new SettingsFragment()).commit();
    }
}

Activity XML (activity_settings.xml)

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.mypackage.MyActivity">

    <include
        layout="@layout/app_bar" />

    <FrameLayout
        android:id="@+id/pref_content"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_below="@id/app_bar"/>

</RelativeLayout>

Fragment (SettingsFragment.java)

import android.preference.PreferenceFragment;
import android.os.Bundle;

public class SettingsFragment extends PreferenceFragment {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        // Load the preferences from an XML resource
       addPreferencesFromResource(R.xml.pref_settings);
   }

}

Fragment XML (pref_settings.xml)

<?xml version="1.0" encoding="utf-8"?>

<PreferenceScreen
xmlns:android="http://schemas.android.com/apk/res/android">

    <CheckBoxPreference
        android:key="pref_checkbox"
        android:title="Title"
        android:summary="Summary"
        android:defaultValue="false"/>

</PreferenceScreen>

Toolbar (app_bar.xml)

<?xml version="1.0" encoding="utf-8"?>

<android.support.v7.widget.Toolbar xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:id="@+id/app_bar"
android:background="@color/primaryColor"
android:elevation="3dp"
app:theme="@style/ThemeOverlay.AppCompat.Dark" />

change this:

    getFragmentManager().beginTransaction().replace(android.R.id.content, new SettingsFragment()).commit();

to:

    getFragmentManager().beginTransaction().replace(R.id.pref_content, new SettingsFragment()).commit();