buffet: Move ShillClient to buffet ShillClient is brillo specific code and should not be in libweave. BUG=brillo:1249 TEST='FEATURES=test emerge-gizmo buffet' Change-Id: Ib44f3621519eb9de4cafae3042117b71de413846 Reviewed-on: https://chromium-review.googlesource.com/290100 Reviewed-by: Vitaly Buka <vitalybuka@chromium.org> Commit-Queue: Vitaly Buka <vitalybuka@chromium.org> Tested-by: Vitaly Buka <vitalybuka@chromium.org>