It's useful to track the flags for IPv4 addresses too.

- we might want to sort IPv4 addresses according to whether they
  are permanent or dynamic.

- later we want to set IFA_F_NOPREFIXROUTE also for IPv4 addresses.
  While the ability to "set" a flag doesn't necessarily require that we
  also keep the flags present in NMPlatformIP4Address, it is more consistent.