Allow ephemeral users on the split-system-user systems only.

BUG: 27143201

Change-Id: I37f3ca7366648dbf07df39a7a972857e0ff78a9a
diff --git a/services/core/java/com/android/server/pm/UserManagerService.java b/services/core/java/com/android/server/pm/UserManagerService.java
index 788c44a..e7465c0 100644
--- a/services/core/java/com/android/server/pm/UserManagerService.java
+++ b/services/core/java/com/android/server/pm/UserManagerService.java
@@ -1833,6 +1833,11 @@
                         return null;
                     }
                 }
+                if (!UserManager.isSplitSystemUser() && (flags & UserInfo.FLAG_EPHEMERAL) != 0) {
+                    Log.e(LOG_TAG,
+                            "Ephemeral users are supported on split-system-user systems only.");
+                    return null;
+                }
                 // In split system user mode, we assign the first human user the primary flag.
                 // And if there is no device owner, we also assign the admin flag to primary user.
                 if (UserManager.isSplitSystemUser()