fix(datetime): setting date async updates calendar grid

[liamdebeasi]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
  • Some docs updates need to be made in the ionic-docs repo, in a separate PR. See the contributing guide for details.
• Build (npm run build) was run locally and any changes were pushed
• Lint (npm run lint) has passed locally and any fixes were made for failures

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Issue URL: resolves #25776

Setting value programmatically when no value was initially selected was not highlighting the correct day because of a defect in how we update highlightActiveParts.

activeParts was originally designed to default to today's date when no value was set. This had the side effect of today's date being highlighted, indicating that it was selected, even if value === undefined.

This was confusing, and we changed the behavior in #25151 so that today's date was outlined when it was not explicitly selected. However, we opted not to change the underlying behavior of activeParts defaulting to today. This behavior is what is ultimately responsible for the reported issue. (In other words, yes highlightActiveParts is preventing the datetime from working as expected, but highlightActiveParts exists because activeParts is not working correctly)

activeParts was designed to represent dates that had been selected by the user or today's date. Since defaulting to today's date is a behavior we are actively trying to avoid with highlightActiveParts, activeParts should no longer default to today's date.

What is the new behavior?

• activeParts now defaults to [] instead of today's date
• I added a helper utility so that elements of datetime can still have today's date as a fallback.
• Remove highlightActiveParts logic since it is no longer needed.

Does this introduce a breaking change?

• Yes
• No

Other information