libweave: Move CommandObserver into public libweave interface Clients will need to listen for Command changes. BUG=brillo:1245 TEST='FEATURES=test emerge-gizmo buffet' Change-Id: I1ba7339127f9d2789a0e09559a875cdc4ecb8df2 Reviewed-on: https://chromium-review.googlesource.com/287069 Tested-by: Vitaly Buka <vitalybuka@chromium.org> Reviewed-by: Alex Vakulenko <avakulenko@chromium.org> Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>