Fix crashes when removing work profile

This isn't specific to work profile. When multiple user
deletions happen in sequence, a race causes a dying user
to lose permissions prematurely. This fix delays removal of
user state until the user is completely cleaned up and all the
processes have been killed.

Bug: 23178833
Change-Id: I1636bc2022416359a25f19a3f65d113c05289cd3

services/core/java/com/android/server/pm/UserManagerService.java

@@ -403,9 +403,10 @@ public class UserManagerService extends IUserManager.Stub {
         return ui;
     }
 
+    /** Called by PackageManagerService */
     public boolean exists(int userId) {
         synchronized (mPackagesLock) {
-            return ArrayUtils.contains(mUserIds, userId);
+            return mUsers.get(userId) != null;
         }
     }