Android artifacts (GeckoView AARs, GeckoViewExample (and Fennec) APKs)
require native libraries (`libxul.so`) and an omnijar (`omni.ja`).
These are produced by `mach package` (really, the `stage-package`
target). Engineers essentially never want a build without a package
for mobile/android. This adds mobile/android-only tiers that run
`mach package` and then `mach android assemble-app`. The latter
consumes `libxul.so` and `omni.ja` to produce _all the things_
relevant to GeckoView engineers.
Differential Revision: https://phabricator.services.mozilla.com/D41450
--HG--
extra : moz-landing-system : lando
