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/packages/SystemUI/src/com/android/systemui/pip/phone/PipManager.java b/packages/SystemUI/src/com/android/systemui/pip/phone/PipManager.java
index 7b8d27e..43cfa32 100644
--- a/packages/SystemUI/src/com/android/systemui/pip/phone/PipManager.java
+++ b/packages/SystemUI/src/com/android/systemui/pip/phone/PipManager.java
@@ -16,7 +16,7 @@
 
 package com.android.systemui.pip.phone;
 
-import android.app.ActivityManagerNative;
+import android.app.ActivityManager;
 import android.app.IActivityManager;
 import android.content.Context;
 import android.view.IWindowManager;
@@ -44,7 +44,7 @@
      */
     public void initialize(Context context) {
         mContext = context;
-        mActivityManager = ActivityManagerNative.getDefault();
+        mActivityManager = ActivityManager.getService();
         mWindowManager = WindowManagerGlobal.getWindowManagerService();
 
         mMenuController = new PipMenuActivityController(context, mActivityManager, mWindowManager);