manager: make manager owns sipvoiplink

The current design describes SIPVoIPLink as a Singleton. This cause
some behaviors where the link is destroyed then immediately created
or vice-versa and its creation/destruction can't really be planned.
This design was made to allow multiple Managers to exist. However,
we never do that, so let's keep it simple, there is only one
Manager and all accounts needs that SIPVoIPLink

This patch change this behavior and the voip link is now owned
by the manager.

Change-Id: I248e41742d342cf452a5503f532fe5ab862166e6