Bluetooth: Refactor internal bt_conn_add() API

Delegates connection object to new connection when there exists
in pool of available connections an object with zeroed destination
LE address.
When there's no reference to the active connection object resets its
peer LE address member.
Replaces routine's input connection handle parameter with LE address.
Moves to the caller setting connection state, connection handle and
call to bt_l2cap_update_conn_param() since connection handle needs
to be valid when such update going to be made.

Change-Id: I81743a915da6cb008f9593dd1940c186a357be6b
Signed-off-by: Arkadiusz Lichwa <arkadiusz.lichwa@tieto.com>