Reduce jank in the wifi detail status page.

Currently, when anything changes, the wifi detail status page
removes and then redraws all IP address information. This causes
the whole screen to flicker. Instead, only add and remove things
when they actually change.

In order to do this, convert the IPv6 addresses from a list of
Preference objects to a single newline-separated text field.
This removes the need to keep track of addresses as they are
added and deleted, and also looks a bit better.

Also, minor correctness fixes:
- Get the gateway from the default route, not from the last route
  with a non-null gateway.
- Get the IPv4 subnet mask from the IPv4 address prefix, not from
  the last route with prefix length > 0.

Bug: 62171690
Test: make -j64 RunSettingsRoboTests
Test: IP information does not flicker when signal strength changes
Change-Id: Ia9f2a277e53a2800407ae327701c5b95a9eec20a