jamiaccount: fix call hangup when removing the account

When an instance of JamiAccount class is destroyed, an attempt
is made  in the destructor to hangup all existing calls by
calling the hangupCalls() method. However, many methods used in
the process (such as terminateSipSession) uses a weak_ptr to
get the account instance, which is not accessible anymore.
The call to hangupCalls() is now called before the JamiAccount
destructor is called.

Gitlab: #627

Change-Id: I26e603a88c5074414d88e59d005d95a952c51d5e