Move Inet[4]AddressUtils to libs/net

The classes should not be picked up from frameworks/base, as they are
part of several mainline modules.

Also refine comments in DhcpResults following feedback in previous
change.

Bug: 151052811
Test: m; manual: flashed, wifi and telephony working
Test: atest NetworkStackCoverageTests
Change-Id: I7074651c6a2a7a6b11bcf13cc4bb03833d7d655f