This rewrites imports of various mojom.dart files from the Flutter
engine repo to instead import normal-looking dart files from the
(new) flutter_services package. This package handles exporting the
correct symbols from generated code wherever that may live.
Includes an engine roll to 3551e7a48e2e336777b15c7637af92fd7605b6c5
which contains the new flutter_services package.
Previously, hello_world.dart was an interactive circle. I've moved that
to touch_input.dart. We should eventually harmonize the touch input
examples at all the layers.
