[Xamarin.Android.Tools.Bytecode] NRT Support

[jonpryor]

Context: 3226a4b
Context: 7068f4b

Enable C# 8 Nullable Reference Types for
Xamarin.Android.Tools.Bytecode.dll.

Fix various warnings emitted by string globalization code analyzers.

[jpobst]

Enable code analyzers for string globalization.

I am curious about this. Nothing in this PR changes the enabled code analyzers. It looks like this fixes the issues that would be raised by CA1307 onnet6.0, which we explicitly disabled in 7068f4b because the needed overloads are not available on netstandard2.0.

[jonpryor]

Nothing in this PR changes the enabled code analyzers.

Correct, but an "intermediary" version of this commit didn't build for netstandard 2.0, but only net6.0, and when $(TargetFramework)=net6.0, lots of CA1307 warnings were emitted, until I fixed them or silenced them.

Also "funny" is the difference between netstandard2.0 & net6.0 around string.IsNullOrEmpty(): net6.0 appears to have [NotNullWhen(false)], while netstandard2.0 doesn't, hence the need for things like:

if (path == null || string.IsNullOrEmpty (path))

'cause without path == null, netstandard2.0 builds would emit CA1307 warnings.