Read only versions of read only flag (ag/28012390.) so the user serial number...

Read only versions of read only flag (ag/28012390.) so the user serial number can be read before flags are initialized.

Change-Id: I7cb811c81601129d4b8508a0b8bc379e96a6c35c
Flag: android.multiuser.cache_user_serial_number_read_only
Bug: 353134536
Test: N/A