Create fat .aar for geckoview

Downstream consumers like android-components and fenix use fat .aar files. We need to create them out of ouf per-arch ones. https://bugzilla.mozilla.org/show_bug.cgi?id=1508976 is the bug where this got implemented on Mozilla's side.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information