Add demo mode to dev options

    <!-- [CHAR LIMIT=25] Bluetooth is disabled. -->

    <string name="bluetooth_disabled">Disabled</string>

    <!-- [CHAR LIMIT=60] Name of dev option called demo mode -->

    <string name="demo_mode">Demo mode</string>

</resources>

        android:key="ota_disable_automatic_update"

        android:title="@string/ota_disable_automatic_update" />

    <PreferenceScreen

        android:key="demo_mode"

        android:title="@string/demo_mode">

        <intent android:action="com.android.settings.action.DEMO_MODE" />

    </PreferenceScreen>

    <PreferenceCategory android:key="debug_debugging_category"

            android:title="@string/debug_debugging_category">