It may be useful to know that starting from Android 8.0 (API level 26) you can use a custom font in XML.
You can apply a custom font to the entire application in the following way.
Put the font in the folder res/font
.
In res/values/styles.xml
use it in the application theme.
<style name="AppTheme" parent="{whatever you like}">
<item name="android:fontFamily">@font/myfont</item>
</style>