Promote enterprise.networkingAttributes API to stable

Moving the newly added enterprise.networkingAttributes API to stable.
Adding documentation for this API and removing the Imprivata
whitelisting for networking.onc API will come as follow-up CLs.

Bug: 1081333
Change-Id: Idcc21f2177faae5cc86d4d72056f9fae72f3cc75
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2250598
Auto-Submit: Alexander Hendrich <hendrich@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#780130}

chrome/common/extensions/api/_permission_features.json

@@ -275,12 +275,12 @@
     ]
   }],
   "enterprise.networkingAttributes": [{
-    "channel": "dev",
+    "channel": "stable",
     "platforms": ["chromeos"],
     "extension_types": ["extension"],
     "location": "policy"
   }, {
-    "channel": "dev",
+    "channel": "stable",
     "extension_types": ["login_screen_extension"],
     "location": "policy",
     "platforms": ["chromeos"],