Add SSA test file to the demo app

I wrote this myself

PiperOrigin-RevId: 281942685

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

@@ -600,6 +600,13 @@
         "subtitle_uri": "https://storage.googleapis.com/exoplayer-test-media-1/ttml/netflix_ttml_sample.xml",
         "subtitle_mime_type": "application/ttml+xml",
         "subtitle_language": "en"
+      },
+      {
+        "name": "SSA/ASS position & alignment",
+        "uri": "https://storage.googleapis.com/exoplayer-test-media-1/gen-3/screens/dash-vod-single-segment/video-avc-baseline-480.mp4",
+        "subtitle_uri": "https://storage.googleapis.com/exoplayer-test-media-1/ssa/test-subs-position.ass",
+        "subtitle_mime_type": "text/x-ssa",
+        "subtitle_language": "en"
       }
     ]
   }