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

services/core/java/com/android/server/am/OomAdjuster.java

@@ -394,6 +394,10 @@ public class OomAdjuster {
         mProcessGroupHandler = new Handler(adjusterThread.getLooper(), msg -> {
             final int pid = msg.arg1;
             final int group = msg.arg2;
+            if (pid == ActivityManagerService.MY_PID) {
+                // Skip setting the process group for system_server, keep it as default.
+                return true;
+            }
             if (Trace.isTagEnabled(Trace.TRACE_TAG_ACTIVITY_MANAGER)) {
                 Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "setProcessGroup "
                         + msg.obj + " to " + group);