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

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