Make launcherSearchProvider API white-listed on stable and public on dev.

BUG=440649

Review URL: https://codereview.chromium.org/1139723002

Cr-Commit-Position: refs/heads/master@{#329381}

chrome/common/extensions/api/_permission_features.json

@@ -538,7 +538,7 @@
     "location": "component"
   }],
   "launcherSearchProvider": {
-    "channel": "dev",
+    "channel": "stable",
     "extension_types": ["extension", "platform_app"],
     "platform": ["chromeos"],
     "whitelist": [