wifi: guess metered flag based on Network Cost information element

Network Cost [1] is a vendor-specific information element defined by
Microsoft and used to advertise the cost of Wi-Fi networks to clients.

We can use it together with the ANDROID_METERED mechanism to
automatically set the metered flag on the device.

[1] https://docs.microsoft.com/en-us/windows-hardware/drivers/mobilebroadband/network-cost-information-element

#200
17 jobs for bg/wifi-network-cost-ie in 70 minutes (queued for 2 seconds)