[System] Use 'ObjCRuntimeInternal' as the namespace instead of 'ObjCRuntime'. (#4820)

[System] Use 'ObjCRuntimeInternal' as the namespace instead of 'ObjCRuntime'. (#4820)

Use 'ObjCRuntimeInternal' as the namespace instead of 'ObjCRuntime' to avoid
these compiler-warnings when building xamarin-macios assemblies (which defines
the same types in the ObjCRuntime namespace, and to whom System.dll has an
InternalsVisibleTo attribute):

> xamarin-macios/src/build/watch/watch/Foundation/NSExtensionRequestHandling.g.cs(28,49): warning CS0436: The type `ObjCRuntime.INativeObject' conflicts with the imported type of same name'. Ignoring the imported type definition
> xamarin-macios/src/ObjCRuntime/INativeObject.cs(6,19): (Location of the symbol related to previous warning)
> xamarin-macios/_ios-build/Library/Frameworks/Xamarin.iOS.framework/Versions/git/lib/mono/Xamarin.WatchOS/repl/System.dll (Location of the symbol related to previous warning)