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: e790874a

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.