Use SQL_DEFAULT_TXN_ISOLATION to get getDefaultTransactionIsolation

Change-Id: I7f4785088151490ae1d55d2bb78bce28357d5c45
Reviewed-on: https://gerrit.libreoffice.org/8863
Reviewed-by: Lionel Elie Mamane <lionel@mamane.lu>
Tested-by: Lionel Elie Mamane <lionel@mamane.lu>