Add kill switch mechanism

This code implements the kill switch mechanism in the extension. This is
explained in //src/killSwitch/README.md and in the design doc:
https://docs.google.com/document/d/1O5YV6_WcxwrUyz-lwHOSTfZ3oyIFWj2EQee0VuKkhaA/edit.

Bug: twpowertools:64

Change-Id: Ia993c78035bba7038aafd53d156f20954217e86f
diff --git a/templates/manifest.gjson b/templates/manifest.gjson
index 8a9fe9d..957193f 100644
--- a/templates/manifest.gjson
+++ b/templates/manifest.gjson
@@ -52,7 +52,8 @@
 #if defined(CHROMIUM || GECKO)
     "https://support.google.com/*",
 #endif
-    "storage"
+    "storage",
+    "alarms"
   ],
 #if defined(CHROMIUM_MV3)
   "host_permissions": [