chromeos_power: change some signals to methods When libcros talks to powerd, it should use method_calls to avoid broadcasting dbus signals to the whole system. BUG=chromium-os:20115 TEST=none Change-Id: Iecf16a9440eac4809267576eef2a218de6953b06 Signed-off-by: Simon Que <sque@chromium.org> Reviewed-on: http://gerrit.chromium.org/gerrit/8094 Reviewed-by: Satoru Takabayashi <satorux@chromium.org>