Add release channel device policy to libpolicy.

BUG=chromium-os:17015
TEST=Added one new test to check that the method returns false when
the release channel is not specified in the policy.

Change-Id: I3c8b4ea9d7d21b685802bcaf60b4394f91709756
Reviewed-on: http://gerrit.chromium.org/gerrit/5068
Tested-by: Julian Pastarmov <pastarmovj@chromium.org>
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
5 files changed