Don't try to handle callbacks until mSystemReady is set.

Bug: 160855861
Bug: 161547406
Test: atest ./hostsidetests/net/src/com/android/cts/net/HostsideRestrictBackgroundNetworkTests.java
Change-Id: Icf570122cae0d9b03319d16df913904bab8ca18d

services/core/java/com/android/server/net/NetworkPolicyManagerService.java

@@ -5288,6 +5288,9 @@ public class NetworkPolicyManagerService extends INetworkPolicyManager.Stub {
         @Override
         public void onTempPowerSaveWhitelistChange(int appId, boolean added) {
             synchronized (mUidRulesFirstLock) {
+                if (!mSystemReady) {
+                    return;
+                }
                 mLogger.tempPowerSaveWlChanged(appId, added);
                 if (added) {
                     mPowerSaveTempWhitelistAppIds.put(appId, true);