Adds a step to the updater to verify that the new .flx package is signed and
untampered.

Each .flx contains a signed manifest file. The manifest contains a SHA-256 hash
of the .flx contents. See bundle.dart for a description of the new .flx format.

Dart code now supports the full flow that the C++ code used to: version check,
download, replace app bundle. Bonus: the Dart code is much easier to follow,
thanks to async/await!

This is part 2 of a 3-part change. The first part added new mojom
interfaces, PathService and UpdateService, to the sky_services package.