Artifacts from local engine builds will be found based on the --local-engine flag

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.

* fix analysis errors

* review comments; fix test

* re-add an export for debugPrint

* rework flutter run

* fix npe with --debug-port

* connect to obs and exit when that conneciton closes

* update todos

Add a command that can capture a Skia picture of a Flutter frame and send it to a file or skiaserve debugger (#3165)

All artifacts for iOS are universal.

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.