Android support for API >= 26 (8, 8.1 and 9)

I finally managed to merge all the required changes into Kinetic branch, together with PRs to rosjava_build_tools and other related repositories.

The PR with the Maven artifacts is open but not yet merged here. Please feel free to try it out and open new issues if necessary; I’ll try to tackle them as soon as I can. Bloom release is also pending.

Cheers, and happy new year!