This adds a flutter_services package responsible for exposing the Dart
interfaces exported by the engine in a uniform fashion, regardless of
how and where the generated code for the interfaces are organized.

We only need this target for preparing the Dart packages for distribution now.

We should eventually rename it to something more sensible, but that's a more
disruptive change. This patch just moves the source code to somewhere easier to
find.

Updating base turns out to be more complicated than I thought.

This reverts commit daa822aa.
This reverts commit c38428c7.
This reverts commit 4b73a8a0.

These build definitions no longer work with the current version of FNL. Once
FNL is ready, we'll add new build definitions that work for it again.

* Add some missing dependecies to sky_services
* Create a script that excludes unnecessary files when preparing the package for release

* Include all the artifacts required to build an APK package
* In addition to the zip, some files will be uploaded separately:
  * libflutter_library.so (large debug symbol library)
  * flutter.mojo (needs to be downloadable from the target device)

We now use flutter.mojo instead.

With this change, each time sky_viewer.mojo is uploaded to Google
Storage (both for Linux and Android), debugging symbols are also
uploaded in a symbols/ subdirectory.

Due to a bad merge, we weren't actually building a distributable version of
the sky_services package.

We still have a few tests failing, I believe
due to different font metrics.  Otherwise this works!

R=abarth@google.com

This CL makess big_red_button.py work for the sky, sky_engine, sky_services
packages.

Fixes Mac builds

We now just symlink the lib directory into the packages directory in the output
directory.

This CL introduces a SkyShell.apk that is separate from the SkyDemo.apk that we
upload to the store to show our demos.  The SkyShell.apk is just an empty shell
that can run Sky applications on Android.

Android and Linux builds.

Also fixes a couple of toolchain selection issues.

We were missing a dependency.

Instead of using a collection of Python scripts to generate the artifacts we
want to distribute from this repository, we should just use the build system.
This CL adds a top-level |dist| target that creates these artifacts in a "dist"
directory of the build output.