[FR] unchecked "prebuilt" library type

[DanAlbert]

Similar to #676

The fix for https://issuetracker.google.com/120166563 is going to require the ability to import modules that are not owned by ndk-build but also are not yet present on disk (have missing "sources"), since Android Studio needs to be able to run ndk-build in a "configure" mode even though ndk-build has no such mode.

We can fake this well enough by adding a new library type that behaves identically to PREBUILT_* but without the safety net. This way PREBUILT_* can still help by failing-fast but prefab can generate more permissive build scripts that fail late.

Open to ideas on the name, but I was thinking EXTERNALLY_BUILT_SHARED_LIBRARY.

[alexcohn]

This is exactly what I proposed few years ago, isn't it?

[DanAlbert]

We discussed it here: #676 (comment)

(Apparently I'd subconsciously remembered that because I even picked the same name despite not remembering it)

This is forked from the other bug because the other bug was pushing hard for completely removing the validation, which we will not be doing.

[DanAlbert]

https://android-review.googlesource.com/c/platform/ndk/+/1833066 does this. I opted for LOCAL_ALLOW_MISSING_PREBUILT rather than a separate module type because it was easier to implement it that way, but it'd be easy to make a new module type that just wraps the old one and sets the property automatically if anyone thinks that's worthwhile.

[alexcohn]

I opted for LOCAL_ALLOW_MISSING_PREBUILT

In my humble opinion, this is good enough, but preferably AGP should be taught to recognize the case when this is required to resolve inter-module dependencies in a multi-module Android project, and set the flag for you.

[DanAlbert]

In practice no one will ever need to set that manually. Prefab will do it for you. The only reason anyone would ever need to use it explicitly would be rolling their own inter-module dependencies, which there isn't much reason to do.