manager: fix newOutgoingCall method

Manager::newOutgoingCall() does not handle case where
a valid account is found (no prefered given) but not usable,
and many accounts are possible.
They are not tested.
This patch fixes this situation and also reduce cyclomatic
complexity by factoring the account search loop.

Change-Id: I505d082f4ca5d7756fe1f257e1767696dc41a2bf