BHV-14650: Convert back to JS date when child picker value changes.

### Issue
When specifying a locale with a non-Gregorian calendar, incrementing/decrementing the year picker can cause an incorrect year value to appear. This is due to the fact that, when the year picker's value changes, the value of the `moon.DatePicker` is set to a Gregorian date via the JavaScript `Date` object. This results in the locale date being calculated using the locale-specific year as a Gregorian year, which causes an incorrect year shift.

### Fix
We retrieve the Gregorian value of the locale date and set this as the current value.

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>