chore(ci): fix failing `deploy-code` job

In #17114, the `deploy-code` CI job was modified to also run some
commands via `yarn`. It turns out that this breaks on CI, because the
`deploy-code` job uses the `cloud-sdk` executor that does not have
`yarn` installed. ([Example failure][1])

This commit fixes this by splitting the `deploy-code` job into two jobs:
- `deploy-code-files` uses the `cloud-sdk` and uploads the files to
  Google Cloud.
  (This is essentially the `deploy-code` job from before #17114.)
- `deploy-code-firebase` uses the `default` executor (which has `yarn`
  installed) and deploys to Firebase.
  (This essentially includes the new bits added in #17114.)

[1]: https://circleci.com/gh/angular/angular.js/2712