Treat process group creation failure due to a dead process as non-fatal

Failure by Zygote to create a process group for newly spawned child is
treated as a fatal error. This is done to avoid leaving children in
Zygote's process group because such relationship can lead to
side-effects, like freezing Zygote when the child's group is being
frozen.
However, if the child died before it could be added into a process
group, there is not danger of leaving it in Zygote's group, therefore
such failure does not have to be fatal and crash system_server.
Check for this situation and when createProcessGroup() fails because
the child is dead, just log the error and keep going.

Bug: 270103958
Change-Id: I6646fbb85162da9087a9e3d7699ef081de277fca
Signed-off-by: Suren Baghdasaryan <surenb@google.com>