Fieldtrial config update for AImageReaderMediaPlayer.

Add a new experiment group which is used for finch on stable release.

Bug: 838725
Change-Id: Ib4e140270e4a1aa70cc676aaae3c8eea8ab8abfc
Reviewed-on: https://chromium-review.googlesource.com/c/1300533
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603008}

testing/variations/fieldtrial_testing_config.json

@@ -10,6 +10,12 @@
                     "enable_features": [
                         "AImageReaderMediaPlayer"
                     ]
+                },
+                {
+                    "name": "Use_AImageReaderMediaPlayer_V3",
+                    "enable_features": [
+                        "AImageReaderMediaPlayer"
+                    ]
                 }
             ]
         }