bluez: Set Powered before other properties

object_set_properties sets the properties one by one, but the bluez
interface doesn't allow setting Discoverable or Pairable unless the
adaptor is powered. So before setting those, power it on first.

Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>
Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D2208