Only apply flag when it has been changed

The properies object provided in onPropertiesChanged only contains
flag(s) that have been added/deleted/updated, changing an unrelated flag
in the same namespace will trigger onPropertiesChanged to be executed
and the properties object won't have your flag, even though it mighet be
set in deviceconfig.

Test: It builds
Bug: 144693310
Change-Id: Iae1692456266efb860f875f3df33fee5831f60cd

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

@@ -957,10 +957,12 @@ public class ActivityManagerService extends IActivityManager.Stub
             new DeviceConfig.OnPropertiesChangedListener() {
                 @Override
                 public void onPropertiesChanged(Properties properties) {
-                    mPssDeferralTime = properties.getLong(ACTIVITY_START_PSS_DEFER_CONFIG, 0);
-                    if (DEBUG_PSS) {
-                        Slog.d(TAG_PSS, "Activity-start PSS delay now "
-                                + mPssDeferralTime + " ms");
+                    if (properties.getKeyset().contains(ACTIVITY_START_PSS_DEFER_CONFIG)) {
+                        mPssDeferralTime = properties.getLong(ACTIVITY_START_PSS_DEFER_CONFIG, 0);
+                        if (DEBUG_PSS) {
+                            Slog.d(TAG_PSS, "Activity-start PSS delay now "
+                                    + mPssDeferralTime + " ms");
+                        }
                     }
                 }
             };