iwd: Work around timing when new 802.1x connection activated

Try work around the issue documented by Emil Velikov in

When we mirror an 802.1x connection to an IWD config file and there's an
AP in range with matching SSID, that connection should become available
for activation.  In IWD terms when an 802.1x network becomes a Known
Network, it can be connected to using the .Connect D-Bus method.

However there's a delay between writing the IWD config file and receiving
the InterfaceAdded event for the Known Network so we don't immediately
find out that the network can now be used.  If an NM client creates a
new connection for an 802.1x AP and tries to activate it quickly enough,
NMDeviceIWD will not allow it to because it won't know the network is
known yet.  To work around this, we save the SSIDs of 802.1x connections
we recently mirrored to IWD config files, for an arbitrary 2 seconds
period, and we treat them as Known Networks in that period since in
theory activations should succeed.

The alternative proposed in the !1264 is to drop NMDeviceIWD checks that
there's a Known Network for the 802.1x connection being activated since
IWD will eventually perform the same checks and IWD is the ultimate
authority on whether the profile is IWD-connectable.
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