Update gradle files to work in both app-services and mozilla-central

[mhammond]

Newer version of #6530

There are 3 commits here, mainly as some might be more controversial than others:

• first commit is use appServicesRootDir rather than rootDir; no real functionality changes just a way so gradle works roughly the same in both places.
• second commit moves app-services to use the same shaped Config object as m-c. When app-services is in m-c, we end up using the Config object from m-c - the app-services version of this object is just for use in app-services so the same gradle code works in both places. Given some comments in the source it appears we maybe used to do that?
• Last commit is where some behaviour changes are implemented depending on whether we are in m-c or app-services. In particular, the code checks gradle.hasProperty("mozconfig") if it needs to do something different based on the location. For now, that means things like disabling the cargo-gradle plugin and changing how uniffi-bindgen is run, etc. The implemented behaviour should be correct in app-services, but might be less so in m-c for now (eg, currently we just disable uniffi-bindgen, whereas ultimately we obviously still need to run it, once I work out how!)

[ncalexan]

Nothing here looks particularly likely to cause pain in the future.

I'm generally concerned that we don't have a lot of high-level oversight of the Gradle build system, and that's causing slow builds and awkward development experiences (that look to get worse before they get much better), but that's the world we're in right now and nothing here is likely to make that more than linearly worse.

[mhammond]

Awesome, thanks Nick! I guess I might as well open this for review then!