Use UMA for testing the UmaAndUkmDemographics Finch experiment

Bug: 1023825
Change-Id: I2304c6085c39dfdbbb412b68d7beea017cc8d924
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1911000Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Roger McFarlane <rogerm@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Roger McFarlane <rogerm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714505}

testing/variations/fieldtrial_testing_config.json

@@ -6722,10 +6722,9 @@
             ],
             "experiments": [
                 {
-                    "name": "UKM_Enabled",
+                    "name": "UMA_Enabled",
                     "enable_features": [
-                        "DemographicMetricsReporting",
-                        "UkmReportNoisedUserBirthYearAndGender"
+                        "DemographicMetricsReporting"
                     ]
                 }
             ]