spanner-jdbc: getTransactionIsolation() returns the default isolation level instead of the current transaction's #14968

Follow-up to #14929, based on this review comment.

Connection#getTransactionIsolation() returns the connection's default isolation level, not the isolation level of the current transaction:

connection.createStatement().execute("BEGIN ISOLATION LEVEL REPEATABLE READ");
connection.getTransactionIsolation(); // returns the default level instead of TRANSACTION_REPEATABLE_READ

Expected: it returns the isolation level of the current transaction, and the default level when there is no transaction.