[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 #64361

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

Co-authored-by: Eric Erhardt <[email protected]>

Co-authored-by: Steve Pfister <[email protected]>
Co-authored-by: Steve Pfister <[email protected]>
Co-authored-by: Eric Erhardt <[email protected]>

Author: 4 people

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

@@ -2,6 +2,7 @@
   <assembly fullname="System.Net.Http">
     <type fullname="System.Net.Http.HttpClientHandler">
       <method signature="System.Boolean get_IsNativeHandlerEnabled()" body="stub" value="false" feature="System.Net.Http.UseNativeHttpHandler" featurevalue="false" />
+      <method signature="System.Boolean get_IsNativeHandlerEnabled()" body="stub" value="true" feature="System.Net.Http.UseNativeHttpHandler" featurevalue="true" />
     </type>
   </assembly>
 </linker>