Use D8 desugaring instead of ThreeTenABP

[zhanghai]

https://developer.android.com/studio/write/java8-support-table

Attention: Development on this library is winding down. Please consider switching to Android Gradle plugin 4.0, java.time.*, and its core library desugaring feature in the coming months.

https://github.com/JakeWharton/ThreeTenABP

This will also allow us to use libraries that depend on Java 8.