Adaptation of https://gist.github.com/AlexMeuer/fb38958e1f154a0fef4e9738df83a49c for a single font. No need to add xml attributes for the same font everywhere if you can just swap out all the TextViews with a custom class.

LatoTextView.java

package foo.bar.baz;

import android.content.Context;
import android.graphics.Typeface;
import android.support.annotation.IntDef;
import android.util.AttributeSet;

import com.google.common.collect.ImmutableMap;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * TextView that just uses the Lato font.
 * Adapted from: https://gist.github.com/AlexMeuer/fb38958e1f154a0fef4e9738df83a49c
 */
public class LatoTextView extends android.support.v7.widget.AppCompatTextView {
    private static final String FONTS_PATH = "fonts/";
    private static ImmutableMap<Integer, String> styleMap = ImmutableMap.of(
            Style.NORMAL, "Lato.ttf",
            Style.BOLD, "Lato-Bold.ttf",
            Style.LIGHT, "Lato-Light.ttf"
    );

    public LatoTextView(Context context) {
        super(context);
    }

    public LatoTextView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public LatoTextView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Retention(RetentionPolicy.SOURCE)
    @Target({ElementType.PARAMETER, ElementType.FIELD, ElementType.METHOD, ElementType.LOCAL_VARIABLE})
    @IntDef({Style.NORMAL, Style.LIGHT, Style.BOLD})
    public @interface Style {
        int NORMAL = Typeface.NORMAL;
        int LIGHT = Typeface.ITALIC; // We don't need italics, se we're overriding it with the Light version of the font.
        int BOLD = Typeface.BOLD;
    }

    @Override
    public void setTypeface(Typeface tf, @Style int style) {
        super.setTypeface(Typeface.createFromAsset(
                getContext().getAssets(),
                FONTS_PATH+getFontPathForStyle(style)
        ));
    }

    public void setStyle(@Style int style) {
        setTypeface(null, style);
    }

    private static String getFontPathForStyle(@Style int style) {
        return styleMap.containsKey(style)
                ? styleMap.get(style)
                : styleMap.get(Style.NORMAL);
    }
}