init: fix writepid wrong pid written in file(s)

When child (pid==0) executes fork branch 'pid_str'
will always be 0 writting wrong value to file(s).
Fix by utilizing getpid() to obtain correct pid.

Tracker link provides some insight on why use cases
aren't broken.

Bug: https://code.google.com/p/android/issues/detail?id=200392



Change-Id: I29989365a8bac94502c090918fa30e5cc88eb1ea
Signed-off-by: Anestis Bechtsoudis <anestis@census-labs.com>