This commit makes the following improvements:
- The code is similar to the other interactive form widgets now, with a
clear note for the only difference.
- Calling `getOrCreateValue` unconditionally ensures that choice widgets
always have a value in the annotation storage. Previously we only
inserted a value in the annotation storage when an option matched or
when a selection was changed. However, this causes breakage when
saving/printing because comboboxes, which we don't fully support yet
but are rendered, might not have a value in storage at all. Their
field value might not match any option since it allows the user to
enter a custom value.
- Calling `getOrCreateValue` unconditionally ensures that forms with
choice widgets no longer always trigger a warning when the user
navigates away from the page. This fixes
https://github.com/mozilla/pdf.js/pull/12241#discussion_r474279654
36e149800e