Show user switcher if guest is enabled

even if there's only one user.

Bug: 15549064
Change-Id: Ib9bc5e6cdc6f2655e44ec0a10a701dd39389e648
diff --git a/core/java/android/os/UserManager.java b/core/java/android/os/UserManager.java
index eb3c748..468cfe0 100644
--- a/core/java/android/os/UserManager.java
+++ b/core/java/android/os/UserManager.java
@@ -882,7 +882,9 @@
                 ++switchableUserCount;
             }
         }
-        return switchableUserCount > 1;
+        final boolean guestEnabled = Settings.Global.getInt(mContext.getContentResolver(),
+                Settings.Global.GUEST_USER_ENABLED, 0) == 1;
+        return switchableUserCount > 1 || guestEnabled;
     }
 
     /**