Host tools can be found in the artifact cache directory for the host platform.
If a developer wants to use a local engine build instead, then provide an
--engine-build flag that selects the specific engine build variant.

* rework flutter run

* fix npe with --debug-port

* connect to obs and exit when that conneciton closes

* update todos

Support "flutter refresh" in the Gradle-based sample project

This provides a fast way to iterate on changes to a Flutter app that only
involve updates to Dart code and do not require a full build and install
of the FLX and APK