[Android] Add an about flag for Download Manager UI

testing/variations/fieldtrial_testing_config_android.json

 {
     "AllBookmarks": [
         {
             "disable_features": [
                 "AllBookmarks"
             ],
             "group_name": "Disabled"
         }
     ],
     "AndroidSessionNotifications": [
@@ skipping 111 matching lines @@
             "enable_features": [
                 "DocumentWriteEvaluator"
             ],
             "group_name": "DocumentWriteEvaluatorGroup",
             "params": {
                 "disallowFetchForDocWrittenScriptsInMainFrame": "false",
                 "disallowFetchForDocWrittenScriptsInMainFrameOnSlowConnections": "false"
             }
         }
     ],
+    "DownloadsUi": [
+        {
+            "enable_features": [
+                "DownloadsUi"
+            ],
+            "group_name": "Enabled"
+        }
+    ],
     "EnableMediaThreadForMediaPlayback": [
         {
             "group_name": "Enabled"
         }
     ],
     "ExpectCTReporting": [
         {
             "enable_features": [
                 "ExpectCTReporting"
             ],
@@ skipping 453 matching lines @@
     ],
     "use-new-media-cache": [
         {
             "enable_features": [
                 "use-new-media-cache"
             ],
             "group_name": "Enabled"
         }
     ]
 }