Since [1], a unique id is used for fieldNodes, therefore, the code to
keep track of last update images on the activity view needs to change
also.
Before this commit, we search the write_date on the fieldModes using the
id, and if it's not present, a new one is created using as id
"write_date".
Now, as a unique id is needed, we search using the name, and if it's not
present, a new one is created using as id "write_date_0".
Note that this is already the behavior on kanbanParse.
[1]: https://github.com/odoo/odoo/commit/baebb6a5b05ac8d59501a6071a5513e31a4ca047closesodoo/odoo#118320
Signed-off-by: Luca Vitali <luvi@odoo.com>