Migrate *-ime-thread switches to feature API

testing/variations/fieldtrial_testing_config_android.json

 {
     "AndroidSessionNotifications": [
         {
             "group_name": "Disabled"
         }
     ],
     "AsyncDns": [
         {
             "group_name": "AsyncDnsA"
         },
@@ skipping 105 matching lines @@
             "params": {
                 "disable_auto_hiding_toolbar_threshold": "800"
             }
         }
     ],
     "EnableMediaThreadForMediaPlayback": [
         {
             "group_name": "Enabled"
         }
     ],
+    "ImeThread": [
+        {
+            "enable_features": [
+                "ImeThread"
+            ],
+            "group_name": "Enabled"
+        }
+    ],
     "InvalidationsGCMUpstream": [
         {
             "group_name": "Enabled"
         },
         {
             "group_name": "Control"
         },
         {
             "group_name": "Disabled"
         }
@@ skipping 295 matching lines @@
     ],
     "use-new-media-cache": [
         {
             "enable_features": [
                 "use-new-media-cache"
             ],
             "group_name": "Enabled"
         }
     ]
 }