Update build dependencies for 2019

[Cliabhach]

While I don't see any need to switch directly to Kotlin now, I could see benefits in targeting a more recent version of the Gradle Wrapper (currently at 3.4) and the Android Gradle Plugin (currently at 2.3.2).

In addition, we could see about building against the Androidx 1.0.0 libraries. I'd like to continue to support consumers using pre-28.0.0 versions of the platform, so perhaps we can find a way to build different artifacts for each audience. Keep in mind that the android jetifier can handle translation from Support Library 28.0.0 to Androidx 1.0.0.

[Cliabhach]

I ultimately included a change in Gradle Wrapper from 3.4 to 4.1rc1 in #77, so the Gradle Wrapper upgrade will only need to go from there to 5.x.