fix(Ads): Fix usage of ENABLE-SKIP-FOR in Interstitals (#7652)

See:
https://mailarchive.ietf.org/arch/msg/hls-interest/TfPMm5VcabtNnauJUf7qIVcFXrE/

lib/ads/interstitial_ad_manager.js

@@ -922,7 +922,7 @@ shaka.ads.InterstitialAdManager = class {
               skipOffset = isSkippable ? 0 : null;
             }
           }
-          const enableSkipForValue = skipControl['X-ENABLE-SKIP-FOR'];
+          const enableSkipForValue = skipControl['ENABLE-SKIP-FOR'];
           if (enableSkipForValue instanceof Number) {
             skipFor = parseFloat(enableSkipForValue);
             if (isNaN(enableSkipForValue)) {