[manifest-attribute-codegen] Automatically generate manifest attributes from Android SDK data.

[jpobst]

Fixes #8272
Context: #8729

Today, we do not have an established process for detecting new XML elements and attributes allowed in AndroidManifest.xml and surfacing them to users via our manifest attributes like [ActivityAttribute]. This leads to users having to use manual workarounds until our attributes can be updated in a future .NET version.

Additionally, when we do add new properties to these attributes, it requires manually updating multiple files by hand that must remain in sync, eg:

‎src/Mono.Android/Android.App/IntentFilterAttribute.cs
src/Xamarin.Android.Build.Tasks/Mono.Android/IntentFilterAttribute.Partial.cs

This commit extends our existing manifest-attribute-codegen script that can parse Android SDKs to determine the possible allowed values in AndroidManifest.xml to additionally generate the code needed by both Mono.Android and Xamarin.Android.Build.Tasks so that we can make updates easily and correctly.

This process is driven by metadata.xml (unrelated to Java binding "metadata") which allows process customization and enforces that every available AndroidManifest.xml value has been accounted for. This ensures that we are making explicit decisions for every new value.

This process is documented in build-tools/manifest-attribute-codegen/README.md.

Also removed the ANDROID_* $(DefineConstants) for Xamarin.Android.Build.Tasks as we no longer build separate assemblies for old Android API levels.

Note this commit does not change any existing manifest attributes or the properties they expose. It merely generates what we expose today. We will determine additional properties to expose in a future commit.

[jonpryor]

I'd like some docs for the structure & semantics of metadata.xml. It has <type/> and <element/> attributes; what do they mean? How do they relate? What are the semantics?

For example:

<type
    name="activity"
    namespace="Android.App"
    outputFile="src\Xamarin.Android.NamingCustomAttributes\Android.App\ActivityAttribute.cs"
    usage="AttributeTargets.Class" jniNameProvider="true"
/>

Where does activity come from?

Or for <element/>:

 <element path="activity.exported" type="bool" />

Where does activity.exported come from?

Is there any relation between these values and data/res/values/attrs_manifest.xml?

[jonpryor]

…like here: where do these name values come from?