Tweaks to the build system

Bug 40857: Modified the fat .aar creation file

This is a workaround to build fat .aars with the compiling enviornment
disabled.
Mozilla does not use a similar configuration, but either runs a Firefox
build and discards its output, or uses artifacts build.

We might switch to artifact builds too, and drop this patch, or write a
better one to upstream. But until then we need this patch.

See also https://bugzilla.mozilla.org/show_bug.cgi?id=1763770.

Bug 41458: Prevent `mach package-multi-locale` from actually creating a package

macOS builds need some files to be moved around with
./mach package-multi-locale to create multi-locale packages.
The required command isn't exposed through any other mach command.
So, we patch package-multi-locale both to prevent it from failing when
doing official builds and to detect any future changes on it.