Migrate to AndroidX, upgrade libraries, general cleanup

[vanshg]

As per https://github.com/cymcsg/UltimateRecyclerView/issues/496, (and rehash of https://github.com/cymcsg/UltimateRecyclerView/pull/497 but merging into a different branch) this PR:

• Updates the library to use AndroidX

And in addition:

• Does some general clean up
• Update Gradle version
• Update SDK version to 28
• Update various library versions
• Update ultimaterecyclerview library version to 0.8.0 (since the API does technically change)
• Update sample app version
• Update README with new library version number

[vanshg]

Great thanks @cymcsg! Do you have an idea of when you could get this uploaded to bintray?

[cymcsg]

Already uploaded as version 0.9.0. Thanks for contributing.

[vanshg]

Awesome thank you @cymcsg!