Update usage of ActivityManagerNative.

- Remove references to ActivityManagerProxy.
- Add isSystemReady to ActivityManager.

Bug: 30977067
Test: cts/hostsidetests/services/activityandwindowmanager/util/run-test android.server.cts
      adb shell am instrument -e class com.android.server.am.ActivityManagerTest,com.android.server.am.TaskStackChangedListenerTest \
          -w com.android.frameworks.servicestests/android.support.test.runner.AndroidJUnitRunner
Change-Id: I07390b6124fb1515821f5c0b37baf6ae74adc8fa
diff --git a/services/core/java/com/android/server/NetworkManagementService.java b/services/core/java/com/android/server/NetworkManagementService.java
index e64aa16..5654096 100644
--- a/services/core/java/com/android/server/NetworkManagementService.java
+++ b/services/core/java/com/android/server/NetworkManagementService.java
@@ -46,7 +46,7 @@
 import static com.android.server.NetworkManagementService.NetdResponseCode.TtyListResult;
 import static com.android.server.NetworkManagementSocketTagger.PROP_QTAGUID_ENABLED;
 import android.annotation.NonNull;
-import android.app.ActivityManagerNative;
+import android.app.ActivityManager;
 import android.content.ContentResolver;
 import android.content.Context;
 import android.net.ConnectivityManager;
@@ -958,7 +958,7 @@
                 final int uid = Integer.parseInt(cooked[1]);
                 final byte[] firstPacket = HexDump.hexStringToByteArray(cooked[2]);
                 try {
-                    ActivityManagerNative.getDefault().notifyCleartextNetwork(uid, firstPacket);
+                    ActivityManager.getService().notifyCleartextNetwork(uid, firstPacket);
                 } catch (RemoteException ignored) {
                 }
                 break;