Skip updating the process group of system_server

Previously launching/dismissing any activities within system_server
would update the process group of system_server, those customized
cpuset settings of some threads within system_server would get messed
up because of it.

Now as those remaining activities within system_server are fairly
simple, we could skip updating the process group of system_server,
keep using the existing default one.

Bug: 172245618
Test: Manual - launch/dismiss activities in system_server & cat cpuset
Change-Id: Ia4d58cc4b8fad1c4ff1a52a05c58f9e62b71df30