…ings

Fixes dotnet#7480

* Remove the use of `PreserveAttribute` as we no longer support Xamarin
  Classic in main.
* Add #pragma and code comments to fix certain CS* warnings which are
  emitted by the C# compiler.
* Ignore a bunch of other warnings in the unit test.

Context: dotnet#8529 (comment)

When building a project that uses layout bindings, multiple nullability
warnings are emitted from `LayoutBinding.cs`:

	…/LayoutBinding.cs(16,98): warning CS8625: Cannot convert null literal to non-nullable reference type.
	…/LayoutBinding.cs(22,90): warning CS8625: Cannot convert null literal to non-nullable reference type.
	…/LayoutBinding.cs(16,13): warning CS8618: Non-nullable field 'boundView' must contain a non-null value when exiting constructor. Consider declaring the field as nullable.
	…/LayoutBinding.cs(22,13): warning CS8618: Non-nullable field 'boundActivity' must contain a non-null value when exiting constructor. Consider declaring the field as nullable.
	…/LayoutBinding.cs(35,11): warning CS8600: Converting null literal or possible null value to non-nullable type.
	…/LayoutBinding.cs(37,11): warning CS8600: Converting null literal or possible null value to non-nullable type.
	…/LayoutBinding.cs(79,56): warning CS8602: Dereference of a possibly null reference.
	…/LayoutBinding.cs(79,56): warning CS8603: Possible null reference return.
	…/LayoutBinding.cs(79,56): warning CA1422: This call site is reachable on: 'Android' 21.0 and later. 'Activity.FragmentManager' is obsoleted on: 'Android' 28.0 and later. (https://learn.microsoft.com/dotnet/fundamentals/code-analysis/quality-rules/ca1422)
	…/LayoutBinding.cs(79,56): warning CA1422: This call site is reachable on: 'Android' 21.0 and later. 'FragmentManager.FindFragmentById<T>(int)' is obsoleted on: 'Android' 28.0 and later (This class is obsoleted in this android platform). (https://learn.microsoft.com/dotnet/fundamentals/code-analysis/quality-rules/ca1422)

Update `LayoutBinding.cs` to include nullability annotations.
Now, no warnings are emitted when
`$(AndroidGenerateLayoutBindings)`=true.

The `SuccessfulAndroidXApp` test was failing, as it asserted no
warnings, yet it had warnings:

    …\LayoutBinding.cs(61,56): warning CS8632: The annotation for nullable reference types should only be used in code within a '#nullable' annotations context.
    …\LayoutBinding.cs(61,72): warning CS8632: The annotation for nullable reference types should only be used in code within a '#nullable' annotations context.
    …\LayoutBinding.cs(79,76): warning CS8632: The annotation for nullable reference types should only be used in code within a '#nullable' annotations context.
    …\LayoutBinding.cs(79,95): warning CS8632: The annotation for nullable reference types should only be used in code within a '#nullable' annotations context.
    …\LayoutBinding.cs(88,86): warning CS8632: The annotation for nullable reference types should only be used in code within a '#nullable' annotations context.
    …\LayoutBinding.cs(88,105): warning CS8632: The annotation for nullable reference types should only be used in code within a '#nullable' annotations context.
    …\LayoutBinding.cs(16,74): warning CS8632: The annotation for nullable reference types should only be used in code within a '#nullable' annotations context.
    …\LayoutBinding.cs(22,66): warning CS8632: The annotation for nullable reference types should only be used in code within a '#nullable' annotations context.
    …\LayoutBinding.cs(12,11): warning CS8632: The annotation for nullable reference types should only be used in code within a '#nullable' annotations context.
    …\LayoutBinding.cs(13,7): warning CS8632: The annotation for nullable reference types should only be used in code within a '#nullable' annotations context.
    …\LayoutBinding.cs(14,30): warning CS8632: The annotation for nullable reference types should only be used in code within a '#nullable' annotations context.
    …\LayoutBinding.cs(33,5): warning CS8632: The annotation for nullable reference types should only be used in code within a '#nullable' annotations context.

Slap on a `#nullable enable` so that we can reliably use
nullable reference types within `LayoutBinding.cs`.