Separate build and sign steps for buildApk and buildAppBundles (#462)
Separate build and sign steps for buildApk and buildAppBundles (#462)

* Separate build and sign steps for buildApk and buildAppBundles

- Developers reported the need to generate "unsigned" APKs and
  Bundles, as signing packages is sometimes generated as part of
  a CI or by different teams (see #413).
- This PR splits build and signing steps into two methods and paves
  the way for a next PR that will enable passing a parameter to
  skip the signing step.
- We also move constant file names to `consts`.

* Address review feedback
GoogleChromeLabs/bubblewrap,GoogleChromeLabs/bubblewrap,GoogleChromeLabs/bubblewrap,GoogleChromeLabs/bubblewrap