[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]

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

[jonpryor]

This bit feels like "dead code" as nothing appears to hit this code path; git grep '<enum-definition name' has no matches. Perhaps this is premature?

[jpobst]

This was existing code that I didn't change:

https://github.com/xamarin/xamarin-android/blob/b521487d0226db265baaa83f0f53e2e75497c420/build-tools/manifest-attribute-codegen/manifest-attribute-codegen.cs#L165-L177

I'm not sure what the original intent was, but maybe the SDK used to specify valid values for some attributes?

[jonpryor]

May want to mention that filePath is expected to be similar to $HOME/android-toolchain/sdk/platforms/android-34/data/res/values/attrs_manifest.xml, and thus dir_name would be android-34, and elements + //declare-styleable is coming from attrs_manifest.xml.

[jonpryor]

Why have this #If block in each file instead of having this within e.g. the src/Xamarin.Android.Build.Tasks/Mono.Android/*.Partial.cs files?

[jonpryor]

[manifest-attribute-codegen] Generate custom attribute declarations (#8781)

Fixes: https://github.com/xamarin/xamarin-android/issues/8272

Context: https://github.com/xamarin/xamarin-android/issues/8235
Context: https://github.com/xamarin/xamarin-android/issues/8729
Context: e790874a21ae0846531b473b003dbc598f0f0310

Previously, we did 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.

Additionally, whenever 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](https://github.com/xamarin/xamarin-android/blob/180dd5205ab270bb74bb853754665db9cb5d65f1/src/Mono.Android/Android.App/IntentFilterAttribute.cs#L9)
  * [src/Xamarin.Android.Build.Tasks/Mono.Android/IntentFilterAttribute.Partial.cs](https://github.com/xamarin/xamarin-android/blob/180dd5205ab270bb74bb853754665db9cb5d65f1/src/Xamarin.Android.Build.Tasks/Mono.Android/IntentFilterAttribute.Partial.cs#L14)

The `build-tools/manifest-attribute-codegen` utility (e790874a) has
support to parse Android SDK `attrs_manifest.xml` files, which
specifies what elements and attributes are valid within
`AndroidManifest.xml`.

Update `manifest-attribute-codegen` to do what it's name already
implied: generate code!  It now reads a `metadata.xml` file which
controls which custom attributes to emit, where to emit them, and
what members those custom attributes should have (among other things).
This makes it easier to ensure that code shared by `src/Mono.Android`
and `src/Xamarin.Android.Build.Tasks` are consistent, meaking it
easier to correctly add support for new attributes and/or
attribute members.

Generated file semantics and naming conventions: consider the C# type
`Android.App.ActivityAttribute`.

  * `src\Xamarin.Android.NamingCustomAttributes\Android.App\ActivityAttribute.cs`
    contains the C# `partial` class declaration that can be shared
    by both `src\Mono.Android` and `src\Xamarin.Android.Build.Tasks`.
    This file also contains a `#if XABT_MANIFEST_EXTENSIONS` block
    which is only used by `src\Xamarin.Android.Build.Tasks`.

  * `src/Xamarin.Android.Build.Tasks/Mono.Android/ActivityAttribute.Partial.cs`
    contains the C# `partial` class declaration with code specific
    to `Xamarin.Android.Build.Tasks.dll`.

  * `src/Xamarin.Android.NamingCustomAttributes/Android.App/ActivityAttribute.Partial.cs`
    contains the C# `partial` class declaration with code specific
    to `Mono.Android.dll`.

`metadata.xml` contents and the update process is documented in
`build-tools/manifest-attribute-codegen/README.md`.

Also removed the `ANDROID_*` values from `$(DefineConstants)` for
`Xamarin.Android.Build.Tasks.csproj` 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.