conference: support handRaised per device

Following the new conference protocol and because a user can
join a conference with several devices, the hand raised is per
device and not per account.
This doesn't change any current behaviour, but is more flexible
for multi-devices support in conference and continue the
implementation of the new conference's protocol.

jami-project#1429

Change-Id: Idab49d8d9efd73eda57706c7a77836689d07ddd7