Bluetooth: host: Refactor initiator to common code and initiator state

Refactor the handling of sending the LE Create Connection command and
for whitelist and direct initiator to use the same host state flag
and common handling of the privacy address.
Also simplify the way we check if the procedure has already been started
when application tries to start it again.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>