Improve speed of the CI pipeline

[ironjan]

b8a8bc58aba2d48451df4f5413d2928981c08503 split the CI pipeline into multiple steps. This would be a great idea, if each travis step would run fast. Currently, they take about 20 minutes each.

Reading material:

• https://medium.com/zendesk-engineering/speeding-up-android-builds-on-travis-ci-1bb4cdbd9c62
• https://coderwall.com/p/x8exja/speed-up-travis-ci-build-preparation-time-by-800
• https://github.com/firebase/quickstart-android/pull/289
• https://docs.travis-ci.com/user/speeding-up-the-build/

[ironjan]

This was done as part of a previous pr.