Fixes: https://github.com/xamarin/xamarin-android/issues/7480

Context: e6048334

When [Layout Bindings][0] are enabled, a set of `partial` classes
will be generated which "mirrors" the `.axml` structure in C#.

Consider:

	dotnet new android -n com.example.codebehind

You enable Code-Behind by:

 1. Setting the `$(AndroidGenerateLayoutBindings)` MSBuild property
    to `true`.
 2. Using `android:id` on elements within `.axml` files.

Consider this patch to the above `dotnet new`:

	diff --git a/Resources/layout/activity_main.xml b/Resources/layout/activity_main.xml
	index f949852..c74521c 100644
	--- a/Resources/layout/activity_main.xml
	+++ b/Resources/layout/activity_main.xml
	@@ -8,6 +8,7 @@
	         android:layout_width="wrap_content"
	         android:layout_height="wrap_content"
	         android:layout_centerInParent="true"
	+        android:id="@+id/text"
	         android:text="@string/app_text"
	     />
	 </RelativeLayout>
	diff --git a/com.example.codebehind.csproj b/com.example.codebehind.csproj
	index 3fdbcb5..8190f84 100644
	--- a/com.example.codebehind.csproj
	+++ b/com.example.codebehind.csproj
	@@ -8,5 +8,6 @@
	     <ApplicationId>com.companyname.com.example.codebehind</ApplicationId>
	     <ApplicationVersion>1</ApplicationVersion>
	     <ApplicationDisplayVersion>1.0</ApplicationDisplayVersion>
	+    <AndroidGenerateLayoutBindings>true</AndroidGenerateLayoutBindings>
	   </PropertyGroup>
	 </Project>

The resulting output contains
`$(IntermediateOutputPath)codebehind\Binding.activity_main.g.cs`,
which declares a type based on the `.axml` file name, and a mirror of
the structure:

	namespace Binding {
	    // typename based on `activity_main.xml` filename
	    sealed partial class activity_main : global::Xamarin.Android.Design.LayoutBinding {
	        [global::Android.Runtime.PreserveAttribute (Conditional=true)]
	        public activity_main (Activity client, …);
	        // …

	        // `text` is via `android:id="@+id/text"`
	        public TextView text => …;
	    }
	}

The problem is the use of `PreserveAttribute` in the generated code;
it's been `[Obsolete]` since e6048334 (over two years ago), which
means all projects using Layout Bindings and Layout Code-Behind always
have [CS0618][1] warnings.

Remove the emission of `PreserveAttribute`, so that CS0618 is no
longer generated.

Additionally, within the generated files disable the [CS1591][2] and
[CS8981][3] warnings:

  * CS1591 is around missing XML documentation comments.  These code
    behind files do not have XML documentation comments, so if/when
    `$(DocumentationFile)` is set, these will be emitted for the
    generated code.  We do not currently plan on emitting XML
    documentation comments, so disable this warning.

  * CS8981 is emitted when a type name consists solely of lowercase
    characters.  As the generated Binding types are based on the
    filename -- which is frequently all lowercase -- this warning
    may be emitted.

Finally, add `#nullable enable` to `LayoutBinding.cs` and fix the
nullable reference type warnings.

[0]: https://github.com/xamarin/xamarin-android/blob/2f4e01ec15102dd9cd922cbd833f6482d69512b5/Documentation/guides/LayoutCodeBehind.md
[1]: https://learn.microsoft.com/dotnet/csharp/language-reference/compiler-messages/cs0618
[2]: https://learn.microsoft.com/dotnet/csharp/language-reference/compiler-messages/cs1591
[3]: https://learn.microsoft.com/dotnet/csharp/language-reference/compiler-messages/warning-waves#cs8981---the-type-name-only-contains-lower-cased-ascii-characters