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/accounts/AccountManagerService.java b/services/core/java/com/android/server/accounts/AccountManagerService.java
index 7802576..b5fecfb 100644
--- a/services/core/java/com/android/server/accounts/AccountManagerService.java
+++ b/services/core/java/com/android/server/accounts/AccountManagerService.java
@@ -34,7 +34,6 @@
 import android.annotation.NonNull;
 import android.annotation.Nullable;
 import android.app.ActivityManager;
-import android.app.ActivityManagerNative;
 import android.app.ActivityThread;
 import android.app.AppGlobals;
 import android.app.AppOpsManager;
@@ -3999,7 +3998,7 @@
     public AccountAndUser[] getRunningAccounts() {
         final int[] runningUserIds;
         try {
-            runningUserIds = ActivityManagerNative.getDefault().getRunningUserIds();
+            runningUserIds = ActivityManager.getService().getRunningUserIds();
         } catch (RemoteException e) {
             // Running in system_server; should never happen
             throw new RuntimeException(e);
@@ -4940,7 +4939,7 @@
 
     private int handleIncomingUser(int userId) {
         try {
-            return ActivityManagerNative.getDefault().handleIncomingUser(
+            return ActivityManager.getService().handleIncomingUser(
                     Binder.getCallingPid(), Binder.getCallingUid(), userId, true, true, "", null);
         } catch (RemoteException re) {
             // Shouldn't happen, local.