[FIX] stock_account: avoid negative values after revaluation

When performing a manual stock revaluation, the value difference is distributed
equally among the available stock. This method breaks down however in case of a
devaluation where some items in stock are already valued less than the unit
cost difference: this results in a negative value.

This commit will prevent such devaluations by raising a UserError whenever the
remaining value of a stock.valuation.layer becomes negative. Additionally, after
a revaluation, the standard_price field will now also be updated for fifo valued
products.

opw-3340298

closes odoo/odoo#126157

Signed-off-by: Arnold Moyaux (arm) <arm@odoo.com>