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/pm/UserRestrictionsUtils.java b/services/core/java/com/android/server/pm/UserRestrictionsUtils.java
index 3df13a9..9fe0922 100644
--- a/services/core/java/com/android/server/pm/UserRestrictionsUtils.java
+++ b/services/core/java/com/android/server/pm/UserRestrictionsUtils.java
@@ -23,7 +23,6 @@
 import android.annotation.NonNull;
 import android.annotation.Nullable;
 import android.app.ActivityManager;
-import android.app.ActivityManagerNative;
 import android.content.ContentResolver;
 import android.content.Context;
 import android.os.Binder;
@@ -411,7 +410,7 @@
                         int currentUser = ActivityManager.getCurrentUser();
                         if (currentUser != userId && userId != UserHandle.USER_SYSTEM) {
                             try {
-                                ActivityManagerNative.getDefault().stopUser(userId, false, null);
+                                ActivityManager.getService().stopUser(userId, false, null);
                             } catch (RemoteException e) {
                                 throw e.rethrowAsRuntimeException();
                             }