No custom textview is required!
First, disable the title in the toobar in your java code : getSupportActionBar().setDisplayShowTitleEnabled(false);
Then, simply add a TextView inside the toolbar :
<android.support.v7.widget.Toolbar
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:background="?attr/colorPrimary"
app:popupTheme="@style/AppTheme.PopupOverlay">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/app_name"
android:textSize="18sp"
android:fontFamily="@font/roboto" />
</android.support.v7.widget.Toolbar>