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.
Runs a test app and a driver test simultaneously, then stops the app.
Usage:
```
flutter drive --target=/path/to/test/app.dart
```
This command will look for `/path/to/test/app_test.dart` by
convention. We will expand into other ways of discovering tests in the
future.
