Fix IMA sample ad tag URL

Issue: #3703

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=181947101

demos/main/src/main/assets/media.exolist.json

@@ -540,7 +540,7 @@
       {
         "name": "VMAP pre-, mid- and post-rolls, single ads",
         "uri": "https://storage.googleapis.com/exoplayer-test-media-1/mkv/android-screens-lavf-56.36.100-aac-avc-main-1280x720.mkv",
-        "ad_tag_uri": "https://pubads.g.doubleclick.net/gampad/ads?sz=640x480&iu=/124319096/external/single_ad_samples&ciu_szs=300x250&impl=s&gdfp_req=1&env=vp&output=vast&unviewed_position_start=1&cust_params=deployment%3Ddevsite%26sample_ct%3Dlinear&correlator="
+        "ad_tag_uri": "https://pubads.g.doubleclick.net/gampad/ads?sz=640x480&iu=/124319096/external/ad_rule_samples&ciu_szs=300x250&ad_rule=1&impl=s&gdfp_req=1&env=vp&output=vmap&unviewed_position_start=1&cust_params=deployment%3Ddevsite%26sample_ar%3Dpremidpost&cmsid=496&vid=short_onecue&correlator="
       },
       {
         "name": "VMAP pre-roll single ad, mid-roll standard pod with 3 ads, post-roll single ad",