Fix the problems with Geckoview's merge_aars phase

In tor-browser-build#40446 (closed), we found that Mozilla changed the way they merge the architecture-specific .aar.

We opened a Bugzilla issue about it.

Mozilla folks are willing to uplift a patch written by us... so we should either create one that is better than our current one, or try to produce local artifacts.

The artifact build system seems not to be compatible with the debugging options (accordingly to Firefox's documentation), but we may introduce architecture-specific builds without the merge_aars phase for when we want to create debug builds (tor-browser-build#40116 (closed)). It is desirable, because:

1. they're much faster to produce
2. and merging AARs that have embedded debug symbols is extremely memory-intensive and time-consuming (my PC has 40GB of memory and I've never managed to complete this step, which took more than 1 hour to fail).