Fix Date picker overlay colors aren't applied on selected state (#159203)
## Description
This PR fixes the DatePicker overlay colors for the selected days.
Before this PR, overlays were obscured by the selected day backgound.
This fix simply replaces a DecoratedBox with an Ink to make the overlays
visible.
Combined with https://github.com/flutter/flutter/pull/159072 which fixes
InkWell overlay color resolution related to the selected state, this PR
fixes [Date picker overlay colors aren't applied on
MaterialState.selected
state](https://github.com/flutter/flutter/issues/130586).
Before, no overlay visible for the selected day when hovered, focused,
or pressed:
https://github.com/user-attachments/assets/944d5035-68b2-40da-b606-3e8795229767
After, overlay is visible for the selected day when hovered, focused, or
pressed (color change is slight as defined with M3 defaults):
https://github.com/user-attachments/assets/2627955b-f45a-465f-8eb0-21955ccd8c3e
## Related Issue
Fixes [Date picker overlay colors aren't applied on
MaterialState.selected
state](https://github.com/flutter/flutter/issues/130586).
## Tests
Adds 12 tests.
Updates several existing tests (those tests were looking for a
`DecoratedBox`, make them look for an `Ink`).
8509d78734