Migrate RecyclerRefreshLayout and sample app to AndroidX

[mhelder]

This PR does a conservative migration from Support Library v25 to AndroidX.

The last version of the Android Support Library was v28. It has been deprecated for some time now, and superseded by AndroidX since it became stable in September 2018. Most open source projects have long migrated to AndroidX, but not RecyclerRefreshLayout. This PR changes that.

Migrating to AndroidX also removes the need for RecyclerRefreshLayout to be 'jetified', which is the compile-time migration of support-library-dependent libraries to equivalent AndroidX packages.

The migration is 'conservative' in the sense that it attempts to only migrate the dependencies and imports to AndroidX, without introducing any other project/code changes. I.e. the Android gradle build tools and gradle wrapper versions remain unchanged. However, some changes are unavoidable. The main one being:

• Support Library v26 raised the minSdkVersion for most libraries tot API 14. That's still the minimum supported Android version required for the AndroidX artifacts RecyclerRefreshLayout depends on.