This follows the model set down for EME artifacts:

- a new tier is added that uses a new Python build action to fetch and
  artifacts
- the action unpacks the fetched artifacts and moves specific inputs
  into places expected by the build and packager
- in automation, MOZ_ARTIFACT_TASK* is used to ensure the artifacts
  come from the correct tasks

In this case, the artifact fetching is done entirely in a new Python
build action that internally uses `mach artifact install --job ...`.
The action also verifies that the fetched artifacts are compatible and
that we're not assembling a fat AAR that is nonsensical.  The specific
inputs are not used in the Fennec APK that is produced; they're only
used in the GeckoView AAR that is produced.

The artifact fetching itself required tweaking to fetch only
`target.maven.zip` artifacts and to not unpack them.

The specific inputs used are the native libraries (libs/$ARCH/*.so)
and the architecture-specific preference files ($ARCH/greprefs.js and
defaults/pref/$ARCH/geckoview-prefs.js).  None of these inputs are
impacted by l10n.

Differential Revision: https://phabricator.services.mozilla.com/D31572

--HG--
extra : moz-landing-system : lando