With the release of Android Oreo you can use the support library to reach this goal.
Reference your default font family in your app main style:
<style name="AppTheme" parent="Theme.AppCompat.Light.NoActionBar">
<item name="android:fontFamily">@font/your_font</item>
<item name="fontFamily">@font/your_font</item> <!-- target android sdk versions < 26 and > 14 if theme other than AppCompat -->
</style>
Check https://developer.android.com/guide/topics/ui/look-and-feel/fonts-in-xml.html for more detailed information.