Bug 32418: Allow updates to be disabled via an enterprise policy.

Restrict the Enterprise Policies mechanism to only consult a
policies.json file (avoiding the Windows Registry and macOS's
file system attributes).

Add a few disabledByPolicy() checks to the update service to
avoid extraneous (and potentially confusing) log messages when
updates are disabled by policy.

Sample content for distribution/policies.json:
{
  "policies": {
    "DisableAppUpdate": true
  }
}

On Linux, avoid reading policies from /etc/firefox/policies/policies.json