[FIX] point_of_sale: round product.product price analogous to backend

A rounding issue was resolved in
ee33593351a148d0f678b36fc01e8524fe1452b6. It however introduced
another issue.

Rounding functions (both round_precision in web.utils and float_round
in openerp.tools) are not perfect due to IEEE floating point
limitations. However both should produce the same output given the
same input. An example: rounding 13.95 to 2 digits yields
13.950000000000001.

The additional rounding introduced in
ee33593351a148d0f678b36fc01e8524fe1452b6 on price lead to issues in
certain cases. One example occurs when applying a 90% discount on a
product costing 13.95. The POS will do the following:

> 13.950000000000001 * 0.09999999999999998
1.3949999999999998
> round_pr(1.3949999999999998, .01)
1.4000000000000001

whereas the backend will do (as eg. in sale.order.line)

>>> 13.95 * 0.09999999999999998
1.3949999999999996
>>> round(1.3949999999999996, 2)
1.3900000000000001

Causing a difference of 0.01.

The core of the issue is that in the backend 13.95 is rounded
differently. When a Float gets written to the database doesn't just
pass through the regular float_round. It passes through
_symbol_set_float which truncates characters exceeding the precision.

This implements the same approach in the POS.

opw-715506
Closes #16119

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