Turn off VrShell experiment in testing config.

testing/variations/fieldtrial_testing_config.json

 {
     "AndroidSessionNotifications": [
         {
             "platforms": [
                 "android"
             ],
             "experiments": [
                 {
                     "name": "Disabled"
                 }
@@ skipping 2398 matching lines @@
         }
     ],
     "VrShell": [
         {
             "platforms": [
                 "android"
             ],
             "experiments": [
                 {
                     "name": "Enabled",
-                    "enable_features": [
+                    "disable_features": [
                         "VrShell"
                     ]
                 }
             ]
         }
     ],
     "WebBluetoothBlocklist": [
         {
             "platforms": [
                 "android",
@@ skipping 97 matching lines @@
                 {
                     "name": "Enabled",
                     "enable_features": [
                         "use-new-media-cache"
                     ]
                 }
             ]
         }
     ]
 }