Add fieldtrial testing config for ButterForPayments on Android

Bug: 1078809
Change-Id: I6e0eec4085fc2f87b18b0945174690489ab3f11c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2467906
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Auto-Submit: Marc Treib <treib@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817331}

testing/variations/fieldtrial_testing_config.json

@@ -1304,6 +1304,21 @@
             ]
         }
     ],
+    "ButterForPayments": [
+        {
+            "platforms": [
+                "android"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "AutofillEnableAccountWalletStorage"
+                    ]
+                }
+            ]
+        }
+    ],
     "CSSMatchedPropertiesCacheDependencies": [
         {
             "platforms": [