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.
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.
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.