[FIX] web: stop defaulting on empty kanbans when you should not

1/ Write a form view with a o2m field that has its own inline tree view
(no other view exist in db for the target model)
2/ Open that view in a mobile environment
3/ Instead of having a list view, you get a kanban view. No luck for
you, since the default kanban generated by `models.py` is an empty one,
your records are *invisible* \o/

From what I can tell, the previous condition made no sense:
```
if (there is a list view and a kanban view)  <-- this seems absurd
  then (use the list mode)
elif (there is no list view but a kanban view)
 then (use the kanban mode)
else (use both modes)
```

The first condition seems absurd: it *actively* ignores a kanban view
if there is one. Even better, the list view whose existence it checks
is never there: it checks for a `tree` when it should be checking for a
`list`. In short, the first condition was *never* met, and you ended up
either with exclusive kanban mode or with both modes - even if you had
no existing kanban view for the target model...

This commit moves the normalization of `tree` into `list` before
attempting to process the x2many field's subviews. It clarify the usage
of `tree` vs `list` (cf. only use `list`) which  prevents weird edge
cases like described above.

closes odoo/odoo#36237

Signed-off-by: Adrien Dieudonné (adr) <adr@odoo.com>
Co-authored-by: Adrien Dieudonné <adr@odoo.com>
Co-authored-by: Pierre Paridans <app@odoo.com>

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