init: Fix a race condition in KillProcessGroup()

Multiple tests in CtsInitTestCases, e.g. RebootTest#StopServicesSIGKILL,
can trigger the following race condition:
* A service is started. This involves calling fork() and also to call
  RunService() in the child process. RunService() calls setpgid().
* Service::Stop() is called and calls KillProcessGroup().
  KillProcessGroup() calls kill(-pgid, SIGKILL) before the child process
  has called setpgid(). pgid is the process ID of the child process. The
  kill() call fails because setpgid() has not yet been called.

Fix this race condition by adding a setpgid() call in the parent process
and by waiting from the parent until the child has called setsid() if a
console is attached.

Bug: 213617178
Change-Id: Ieb9e6908df725447e3695ed66bb8bd30e4e38aa9
Signed-off-by: Bart Van Assche <bvanassche@google.com>