Avoid OOM Adj update for processes with fixed adj.

Ignore the oomAdjUpdate request if the target app is
system/persistent process with fixed adj.

Bug: 226385494
Test: atest MockingOomAdjusterTests
Change-Id: I796298cd6877577394960d944112c843f0a637cc

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

@@ -806,7 +806,7 @@ public class OomAdjuster {
      */
     @GuardedBy("mService")
     void enqueueOomAdjTargetLocked(ProcessRecord app) {
-        if (app != null) {
+        if (app != null && app.mState.getMaxAdj() > FOREGROUND_APP_ADJ) {
             mPendingProcessSet.add(app);
         }
     }