Rename ApfGenerator to ApfV4Generator

This change is part of a series of changes that split ApfGenerator into
three classes: AbstractApfGenerator, ApfV4Generator, and ApfV6Generator.

Bug: 319394073
Test: TH
Change-Id: I2d831d5b4f6c1bcf2408f5bfaa86bba2c0a9c3e8