Network Traffic: Consider CLAT interfaces (IPv6)

IPv6-only networks that support 464XLAT use CLAT to access IPv4
addresses, but the traffic that occurs over the CLAT interface is
tracked separately; it is not reflected at all in the stats of the
underlying interface, so prior to this change, IPv4 traffic on
applicable IPv6-only networks is not reflected in the network traffic
monitor, either.

While there is no exposed API to retrieve stacked interfaces, we can
still try to grab the stats for a CLAT interface (which always starts
with "v4-"), just in case it exists, and if so, add in those stats.

Test: Manual: While connected to an IPv6-only network, such as a
T-Mobile cellular data connection, attempt to load https://1.1.1.1


in a browser while the network traffic monitor is enabled. Reload
the page / choose different languages to cause traffic to occur,
and it should be reflected in the traffic monitor.

Issue: calyxos#792
Co-authored-by: Oliver Scott <olivercscott@gmail.com>
Change-Id: Ida768ebe73a47bb06da53aeb7b5c6882f0090e75