* Integrate Spotless in build
* Update CONTRIBUTING.md
* Move improperly placed copyright comments
* Update copyright indent
* Don't format test fixtures
* Run spotlessApply over the whole repo
* Remove checkstyle configs
* Run spotlessCheck on CI
* Migration to AndroidX #324
* Divided versions for androidx libraries and renamed dependencies #324
* Upgrade target sdk version for travis #324
* Migration to androidx part 2 #324
* Fixed spaces #324
Uber's implementation of screenstack is quite complicated. It includes analytics and various forms of (infrequently used) animation customization. I don't want to open source the full implementation because (1) that would be difficult (2) a lot of the functionality doesn't make sense as a general utility.