[Jank] [a11y] Add fieldtrial testing config for a11y experiment.

Adds configuration for OptimizeAccessibilityUiThreadWork flag.

Bug: 1352616
Change-Id: I7c2c9cf3e20c6a556be19a19b8d8b29586f28346
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3884946
Commit-Queue: Yaron Friedman <yfriedman@chromium.org>
Reviewed-by: Peilin Wang <peilinwang@google.com>
Cr-Commit-Position: refs/heads/main@{#1047096}

testing/variations/fieldtrial_testing_config.json

@@ -7079,6 +7079,21 @@
             ]
         }
     ],
+    "OptimizeAccessibilityUiThreadWork": [
+        {
+            "platforms": [
+                "android"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "OptimizeAccessibilityUiThreadWork"
+                    ]
+                }
+            ]
+        }
+    ],
     "OptimizeLayoutsForPullRefresh": [
         {
             "platforms": [