[Xamarin.Android,Build.Tasks] Cannot merge Android Manifest elements from class library

[dellis1972]

Fixes #8262

So if a user wants to distribute an AndroidManifest.xml file along with a NuGet the file is currently NOT included in the library .aar file. So lets include it if one is present. This will allow NuGet authors to set default permissions if need be by simply adding the file to the project directory.

This also fixes an issue in GenerateLibraryResources where if a manifest file does NOT have a package name.

[jonathanpeppers]

Overall looks good, just had the one thought/comment. 👍

[dellis1972]

/azp run

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Azure Pipelines successfully started running 1 pipeline(s).

[jonpryor]

What is the rationale for moving these properties?

[jonpryor]

Draft commit message:

[Xamarin.Android,Build.Tasks] Library .aar includes $(AndroidManifest) (#8273)

Fixes: https://github.com/xamarin/xamarin-android/issues/8262

Context: https://developer.android.com/studio/projects/android-library#aar-contents
Context: fcd7cf8f4200ccae92e333c920613bfec260973b

In fcd7cf8f, we stopped using `@(EmbeddedResource)` within assemblies
to contain Android Resource, Assets, Libraries, etc., and instead
began a `.aar` file.

One of the things never supported in the Classic system was to allow
Library projects to contain `AndroidManifest.xml` files, and package
`AndroidManifest.xml` files into the library `.aar` file for inclusion
in NuGet packages.

Add `$(AndroidManifest)` to `@(_CreateAarInputs)`, so that Library
projects can now specify an `AndroidManifest.xml` file for inclusion
in the library `.aar` file (which in turn should be included in NuGet
packages).

This allows NuGet authors to e.g. set default permissions if need be
by simply adding `AndroidManifest.xml` to the library project dir.

Also, fix an issue in `<GenerateLibraryResources/>` when a manifest
file does NOT have a package name.  Such files should be ignored.

[jonpryor]

@dellis1972: do we have any idea what, if anything, this will do when done broadly?

On the one hand, AndroidManifest.xml is the one required file in .aar files: https://developer.android.com/studio/projects/android-library#aar-contents

The only mandatory entry is /AndroidManifest.xml.

On the other hand, this is still A Change, and I find it hard to know if this could possibly break anything. For example, if someone starts with an App project (dotnet new android), then "changes their mind" and starts using it as a library project. (Is that even a thing? We used to have $(AndroidApplication), but I don't see that in our current template, so how do we know if we're building an App or a Library anymore?)

Said "app now library" project will contain an AndroidManifest.xml, quite possibly the default:

https://github.com/xamarin/xamarin-android/blob/1a309b63345d64356a68805c2e20f1c4fd794e42/src/Microsoft.Android.Templates/android/AndroidManifest.xml

What happens if an app starts getting multiple copies of our default AndroidManifest.xml? Is that a problem?

I'm wary of including a change this close to release when I can't foresee how it will impact the ecosystem.