[fuchsia] Ensure flutter_apps in the same BUILD.gn have separate scratch directories for building the FLX. (#3720)

 - Fix flutter_aot_app template for moved Flutter tool GN label.
 - Add missing dart:io tree-shaking roots.
 - Add example AOT app to catch future breakage.

This allows tools to locate them more easily.

* Remove sky_snapshot sources

* update licenses

Now that the observatory build works in Fuchsia, use dart instead of dart_no_observatory so that dart_no_observatory can be removed from the build.

This will ensure the flx file is rebuilt whenever the manifest is modified.

This is to ensure that the depfile generated by the snapshotter has
the correct target.

We need to pass the FLUTTER_ROOT to the flutter_tools so that it can find
resources such as the schema for flutter.yaml.

The name of the generated packages file now includes the target name.