Bluetooth: Shell: Only auto-connect to unconnected devices

The bt connect and bt connect-name commands should only
attempt to connect to connectable devices that we are not
already connected to.

This commit moves the check for BT_GAP_ADV_PROP_CONNECTABLE
as well as checks that we do not already have a connection
to that device with the current ID.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>