[release/6.0] [mobile] Add extra HttpClientHandler linker substitution (#64852)

[release/6.0] [mobile] Add extra HttpClientHandler linker substitution (#64852)

* [mobile] Add extra HttpClientHandler linker substitution

The linker substitution for HttpClientHandler.IsNativeHandlerEnabled only worked when <UseNativeHttpHandler> in a project was set
to false.  Since the default for MAUI projects is true, this caused us to carry more assemblies than needed.

Fixes https://github.com/dotnet/runtime/issues/64361

* Update src/libraries/System.Net.Http/src/ILLink/ILLink.Substitutions.mobile.xml

Co-authored-by: Eric Erhardt <eric.erhardt@microsoft.com>
Co-authored-by: Steve Pfister <steve.pfister@microsoft.com>
Co-authored-by: Steve Pfister <steveisok@users.noreply.github.com>
Co-authored-by: Eric Erhardt <eric.erhardt@microsoft.com>