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/statusbar/phone/StatusBarWindowManager.java b/packages/SystemUI/src/com/android/systemui/statusbar/phone/StatusBarWindowManager.java
index 4a2d5dc..995f4dd 100644
--- a/packages/SystemUI/src/com/android/systemui/statusbar/phone/StatusBarWindowManager.java
+++ b/packages/SystemUI/src/com/android/systemui/statusbar/phone/StatusBarWindowManager.java
@@ -16,7 +16,7 @@
 
 package com.android.systemui.statusbar.phone;
 
-import android.app.ActivityManagerNative;
+import android.app.ActivityManager;
 import android.app.IActivityManager;
 import android.content.Context;
 import android.content.pm.ActivityInfo;
@@ -64,7 +64,7 @@
     public StatusBarWindowManager(Context context) {
         mContext = context;
         mWindowManager = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);
-        mActivityManager = ActivityManagerNative.getDefault();
+        mActivityManager = ActivityManager.getService();
         mKeyguardScreenRotation = shouldEnableKeyguardScreenRotation();
         mScreenBrightnessDoze = mContext.getResources().getInteger(
                 com.android.internal.R.integer.config_screenBrightnessDoze) / 255f;