Android использует строковый ресурс в файле settings.xml

Попробуйте следующий простой пример:

1) MnnnnActivity.class:-----------------

public class MnnnnActivity extends AppCompatActivity {

private Button b;
private TextView tv;
private SharedPreferences preferences;

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.layout4);

    String preferenceName = getResources().getString(R.string.pref_key);

    preferences = getApplicationContext().getSharedPreferences(preferenceName,
            0);

    b = (Button) findViewById(R.id.b);
    b.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Intent i = new Intent(MnnnnActivity.this , prefAct.class);
            startActivity(i);
        }
    });

    tv = (TextView) findViewById(R.id.tv);

}

@Override
protected void onResume() {
    super.onResume();

    if(preferences != null && tv != null){
        tv.setText("Changed to " + String.valueOf(preferences.getBoolean(getResources().getString(R.string.pref2) , false)));
    }


}
}

2) prefAct.class: ---------

public class prefAct extends AppCompatActivity {

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

    FragmentManager mFragmentManager = getFragmentManager();
    FragmentTransaction mFragmentTransaction = mFragmentManager
            .beginTransaction();
    EditPrefs mEditPrefs= new EditPrefs();
    mFragmentTransaction.replace(android.R.id.content, mEditPrefs);
    mFragmentTransaction.commit();
}
}

3) EditPrefs.class: -------------

public class EditPrefs extends PreferenceFragment implements  SharedPreferences.OnSharedPreferenceChangeListener {

private int preferencesToEdit;

private PreferenceHelper sensor_preferences;

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

    String preferenceName = getResources().getString(R.string.pref_key);

    preferencesToEdit = R.xml.pr;

    PreferenceManager preferenceManager = getPreferenceManager();
    preferenceManager.setSharedPreferencesName(preferenceName);
    preferenceManager.setSharedPreferencesMode(0);

    getActivity().setTitle(getResources().getString(R.string.pref_sensor_action_bar_title));// TODO:change this

    addPreferencesFromResource(preferencesToEdit);

}

@Override
public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, String key) {


    if (findPreference(key) != null) {

        if (findPreference(key).getKey().equals(getResources().getString(R.string.pref2))) {
            Toast.makeText(getActivity(), "Changed to " + sharedPreferences.getBoolean(getResources().getString(R.string.pref2), false), Toast.LENGTH_LONG).show();
        }
    }
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View view = super.onCreateView(inflater, container, savedInstanceState);
    if(view != null) {
        view.setBackgroundColor(getResources().getColor(android.R.color.white));
    }
    return view;
}

@Override
public void onResume() {
    super.onResume();
    getPreferenceScreen().getSharedPreferences().registerOnSharedPreferenceChangeListener(this);
}

@Override
public void onPause() {
    super.onPause();
    getPreferenceScreen().getSharedPreferences().unregisterOnSharedPreferenceChangeListener(this);
}

@Override
public void onDestroy() {
    super.onDestroy();
    getPreferenceScreen().getSharedPreferences().unregisterOnSharedPreferenceChangeListener(this);
}

}

4) layout4.xml: ---------------

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">

<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/b"
    android:layout_marginTop="20dp"
    android:layout_marginBottom="10dp"
    android:layout_gravity="center"
    android:text="Click"/>

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_gravity="center"
    android:id="@+id/tv"
    android:text="x"/>

</LinearLayout>

5) пр.xml: ----------------

<?xml version="1.0" encoding="utf-8"?>
<PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android">

<PreferenceCategory>

    <CheckBoxPreference
        android:key="@string/pref2"
        android:summary="@string/pref_summary_auto_delete"
        android:title="@string/pref_title_auto_delete"
        android:defaultValue="false" />

</PreferenceCategory>


</PreferenceScreen>

6) строковые ресурсы: --------

<string name="pref_key">pref_key</string>
<string name="pref2">pref2</string>
<string name="pref_title_auto_delete">title</string>
<string name="pref_summary_auto_delete">summary</string>

7) Не забудьте включить обе активности в манифест.