iOS Simulator: Attempt 'sideloading' the updated Flutter application if the application runner is already up

* This makes the turnaround times < 2 seconds on the iOS simulator.
* Also bumps the engine to use the new engine required to support the flags.

flutter start's method of finding devices to run the app on is not suitable for flutter drive.

This commit also refactors several tool services to allow mocking in unit tests.