Add flutter_gallery__transition_perf of the new Flutter gallery app

[liyuqian]

It's important to ensure that our new Flutter gallery app (already released in Google play store) is performant. Therefore, the new app should at least have some automated perf test like the old flutter_gallery__transition_perf test to continuously monitor its performance.

[redbrogdon]

Closing this now that the gallery has moved to https://github.com/flutter/gallery.