fix(audience-sdk): gate iOS attribution providers behind AUDIENCE_MOBILE_ATTRIBUTION (SDK-347)

Without this define, iOS could collect ATT status and IDFA at runtime even
when AUDIENCE_MOBILE_ATTRIBUTION was not set, producing a mismatch between
runtime behaviour and the privacy manifest (NSPrivacyTracking = false while
IDFA was being collected). Both platforms now require the define before any
attribution provider is registered, matching the documented two-gate model.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: nattb8

src/Packages/Audience/Runtime/Unity/AudienceUnityHooks.cs

@@ -35,7 +35,7 @@ private static void Install()
             ImmutableAudience.LaunchContextProvider = () => launchProps;
             ImmutableAudience.ContextProvider = () => contextProps;
 
-#if UNITY_IOS && !UNITY_EDITOR
+#if UNITY_IOS && !UNITY_EDITOR && AUDIENCE_MOBILE_ATTRIBUTION
             ImmutableAudience.MobileAttributionProvider = () => SkanRegistration.RegisterIfFirstLaunch();
             ImmutableAudience.MobileAttributionContextProvider = () => AttributionContext.Capture();
             ImmutableAudience.TrackingAuthorizationRequestProvider = () => ATTBridge.RequestAsync();