Add demo app test stream with overlapping SSA subtitles muxed in MKV

This doesn't currently work by default, but upcoming changes in
`TextRenderer` will make it work as expected.

Issue: #10295
PiperOrigin-RevId: 553161628

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

@@ -642,6 +642,10 @@
       {
         "name": "SubRip muxed into MKV",
         "uri": "https://storage.googleapis.com/exoplayer-test-media-1/mkv/android-screens-with-subrip.mkv"
+      },
+      {
+        "name": "Overlapping SSA muxed into MKV",
+        "uri": "https://storage.googleapis.com/exoplayer-test-media-1/mkv/android-screens-with-overlapping-ssa.mkv"
       }
     ]
   },