Pending transfers notifications use locked amount

[manuelwedler]

Describe the bug

The pending transfers notifications use the locked_amount that is returned from the API to show the payment amount. When you do the same payment of 100 tokens twice you would get one notification for a payment of 100 tokens and one for a payment of 200 tokens, as the locked amount increases while both are pending. The API does not return the actual payment amount at the moment. See raiden-network/raiden#5474

When confirming to send the same payment that is already in flight, the locked_amount is used too.