So we can trace the correct performance (first frame latency, worst frame after the first frame), and do the correct screenshot of the first frame.

Previously, it only knows the event when the UI thread finished preparing the first frame. That made us measure the wrong performance metrics, and take the wrong screenshots (blank screens instead of the first frame).

In order to implement this, we'll need to let UI thread and GPU thread communicate through VMService. Currently flutter_tools package has a VMService that flutter_drive could use but flutter_drive can't depend on flutter_tools. Hence we'll do a big yak shave to extract that VMService and put it in a common package that can be used by both flutter_tools and flutter_drive.