Android 8.0 (API 26) introduced new features related to fonts.
1) Fonts can be used as resources.
2) Downloadable fonts.
If you want to use external fonts in your android application, you can either include font files in apk or configure downloadable fonts.
Including font files in APK : You can download font files, save them in res/font filer, define font family and use font family in styles.
For more details on using custom fonts as resources see http://www.zoftino.com/android-using-custom-fonts
Configuring downloadable fonts : Define font by providing font provider details, add font provider certificate and use font in styles.
For more details on downloadable fonts see http://www.zoftino.com/downloading-fonts-android