Use dynamic exclusions for excluding Wi-Fi calling ePDG servers

[ameshkov]

There were some newly reported ISPs we should add, see new comments in this issue:
#2550

However, I don't really like that we have to add new exclusions for every ISP. Instead of that, we should consider excluding just the IP addresses of the EPDG servers of the current cellular provider.

This Android code may be useful for that: https://cs.android.com/android/platform/superproject/+/master:packages/services/Iwlan/src/com/google/android/iwlan/epdg/EpdgSelector.java

In theory, epdg discovery is simple:

ePDG server discovery is simple: all ePDG server names look like epdg.epc.mnc%MNC%.mcc%MCC%.pub.3gppnetwork.org where %MNC% is your provider's Mobile Network Code, and %MCC% is its Mobile Country Code. You can find these values of your provider here: http://www.mcc-mnc.com/. Both values are padded with zeros to the length of 3.

However, some cellular providers use a different domain name for their ePDG, we should figure out why.
#2550 (comment)

// weconnect.globe.com.ph (Globe uses a different domain for their ePDG servers. Similiar to how AT&T does it)
// Globe Philippines Wi-Fi calling

Usually, there's a file like https://github.com/zakooz/SM-G930F_system_etc/blob/master/epdg_apns_conf.xml on the device with EPDG server addresses.

[sfionov]

@ameshkov Does not receiving MCC/MNC requires TELEPHONY permission? It can scare off users imo

[ameshkov]

@sfionov if the implementation requires additional permissions, we shouldn't do it of course. But maybe there's a chance we can deduct it without that, investigation is required:)