Do not purge process groups on system_server startup

Removing all process groups during system_server startup can race with
process group creation from init which results in occasional failure to
create a process group and failure to boot the device.
Move process group purging into init to prevent such a race.

Bug: 228160715
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: I2e50f53c9692424c994c0b4ee6f1e2762632c3bc