[One .NET] UseSystemResourceKeys=true only when linking (#6186)

Fixes: #6185
Context: dotnet/runtime#56990

We should *only* default `UseSystemResourceKeys` to `true` if the
linker is enabled, so users get nice exception messages while
debugging.

If a user desires, they could retain long exception messages in
release builds by setting the value in the `.csproj`:

    <UseSystemResourceKeys>false</UseSystemResourceKeys>

src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.DefaultProperties.targets

@@ -87,7 +87,7 @@
       See: https://github.com/dotnet/sdk/blob/1f544a59270cecb2947e50a01f7056c685b4e319/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Sdk.targets#L389-L392
     -->
     <InvariantGlobalization Condition="'$(InvariantGlobalization)' != 'true'"></InvariantGlobalization>
-    <UseSystemResourceKeys Condition="'$(UseSystemResourceKeys)' == ''">true</UseSystemResourceKeys>
+    <UseSystemResourceKeys Condition="'$(UseSystemResourceKeys)' == '' and '$(PublishTrimmed)' == 'true'">true</UseSystemResourceKeys>
     <HttpActivityPropagationSupport Condition="'$(HttpActivityPropagationSupport)' == ''">false</HttpActivityPropagationSupport>
     <StartupHookSupport Condition="'$(StartupHookSupport)' == ''">false</StartupHookSupport>
     <UseNativeHttpHandler Condition="'$(UseNativeHttpHandler)' == ''">true</UseNativeHttpHandler>