Adding screenshot tests to postsubmit.

Test: atest --host :postsubmit
Bug: 314807258
Change-Id: I7c593520369f308b5678554f4ea0afe335698a96

TEST_MAPPING

@@ -37,6 +37,10 @@
     },
     {
       "name": "SettingsPerfTests"
+    },
+    {
+      "name": "SettingsScreenshotRNGTests",
+      "host": true
     }
   ]
 }