libprocessgroup: fix boot time performance regression

The way processes are accounted in DoKillProcessGroupOnce has been
changed recently, which affects retries in KillProcessGroup. More specifically, initialPid was not counted before and would not
cause a retry with 5ms sleep.
Restore previous behavior to avoid boot time regressions.

Bug: 271198843
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: Ibc1bdd855898688a4a03806671e6ac31570aedf9