iwd: nm_iwd_manager_get() once and save value

Call nm_iwd_manager_get once on NMDeviceIwd creation and save in
priv->manager to avoid using t very often now that we have 5 new call
sites.  The reasoning is explained in
NetworkManager/NetworkManager!1338 (comment 1519684)
56 jobs for iwd-fixes in 27 minutes and 5 seconds (queued for 3 seconds)