Clarify the role of `dev/devicelab` and running on emulators/simulators

The `dev/devicelab` folder is _not_ just for tests running on the device lab, but also tests using device-lab (Dart) test runner to run on an emulator, simulator, or sometimes neither of those (i.e. just running some variant of `flutter build`). We talked and decided all of that was appropriate (enough), but the best thing we can do is at least update `dev/devicelab/README.md` (and probably other docs, TBD), and then give the team a PSA.