[16.0][MIG] account_manual_currency

[Saran440]

In odoo16 remove many onchange in account.move.line change to compute field. So, this PR will change function for support core odoo

• Show currency_rate in line and will change from manual_currency_rate (if use manual currency)
• When add 0 in manual currency rate. it will change to default last rate currency (not allow add rate 0.0)
• Change method _fields_view_get to get_view

[rrebollo]

@Saran440 are U still working on this? How can I help??? Is this a work in progress?? Any clue about the failing test???

[Saran440]

@rrebollo Hi, I fixed some test script. if you tested and it work. please approve this PR.