Add field trial testing configuration for AndroidNightMode

Bug: 925562
Change-Id: Ied102c3e8f15c2ee15f9e4a7eb3d87a2d433e5ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1617084Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Becky Zhou <huayinz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661042}

testing/variations/fieldtrial_testing_config.json

@@ -426,6 +426,21 @@
             ]
         }
     ],
+    "AndroidNightMode": [
+        {
+            "platforms": [
+                "android"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled_20190503",
+                    "enable_features": [
+                        "AndroidNightMode"
+                    ]
+                }
+            ]
+        }
+    ],
     "AndroidSpellChecker": [
         {
             "platforms": [