[FIX] stock_account: correct accounts for dropship return

Before this commit:
When returning a dropshipping, the valuation layers created do not have
the correct accounts on it:
 - Valuation -> Input   for the first SVL
 - Output -> Valuation  for the second SVL

After this commit:
For a dropshipped move, valuation layers have the following chain
of accounts:
 - Input -> Valuation   for the first SVL
 - Valuation -> Output  for the second SVL
Therefore, the return should have it reversed:
 - Output -> Valuation  for the first SVL
 - Valuation -> Input   for the second SVL

Steps to reproduce:
1. Create a dropship product with automated inventory valuation
2. Create a Sales Order, go on the PO and confirm it
3. Set quantities and validate dropshipping
4. Return delivered product (dropship return)

opw-3391174

closes odoo/odoo#136024

X-original-commit: 30de83a5
Signed-off-by: William Henrotin (whe) <whe@odoo.com>