init: Purge empty process groups on zygote restart

When system_server crashes or gets killed, it causes zygote to kill
itself, which in turn leads to killing all processes in the same
process group (all apps). This leaves empty process groups because
system_server is not there to remove them.
Purge empty process groups when init detects zygote death.

Bug: 228160715
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: I0ce27eea28f8713e52033bbec2d5363a7b8ff5db
Merged-In: I0ce27eea28f8713e52033bbec2d5363a7b8ff5db