[NativeAOT] Restore NativeAOT-requires-trimmable build error

The <Error> guarding NativeAOT against a non-trimmable typemap was
dropped when [NativeAOT] Make the trimmable typemap the default was
replayed onto main, because main had wrapped the typemap validation in
the new _ValidateAndroidTypeMapImplementation target. Re-add it there.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: simonrozsival

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets

@@ -2948,6 +2948,8 @@ because xbuild doesn't support framework reference assemblies.
       BeforeTargets="_CheckForInvalidConfigurationAndPlatform">
     <Error Condition=" '$(_AndroidTypeMapImplementation)' != '' and '$(_AndroidTypeMapImplementation)' != 'llvm-ir' and '$(_AndroidTypeMapImplementation)' != 'managed' and '$(_AndroidTypeMapImplementation)' != 'trimmable' "
            Text="Invalid value for _AndroidTypeMapImplementation: '$(_AndroidTypeMapImplementation)'. Valid values are: llvm-ir, managed, trimmable." />
+    <Error Condition=" '$(_AndroidRuntime)' == 'NativeAOT' and '$(_AndroidTypeMapImplementation)' != 'trimmable' "
+           Text="NativeAOT requires _AndroidTypeMapImplementation=trimmable." />
   </Target>
 
   <Target Name="_LinkAssemblies"