Implement method to bring an existing app/browser to the foreground. This would allow to write benchmarks that interact with several apps/browsers within a single user story.

Our current priority is to implement this for Android backends as it is needed for  issue 585760 ; but it would be useful to implement for other platforms too.

Design doc: https://docs.google.com/a/google.com/document/d/1a5-05ecExPcJ1x0z8ufLyhjmEoUBMFcMlzrg3SSyWeY/edit?usp=sharing