BHV-13458: Update DatePicker to properly handle locale changes, both before and after having a value set.

Issue
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When the locale changes, the child picker values are not being updated to reflect this change.

Fix
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The code to compute the current value has been refactored to be shared by both `setupPickers` and `setChildPickers`, and we update the `ilib` locale when we are notified that the locale has changed.

Notes
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This fix might need to be updated, depending on the final fix from https://github.com/enyojs/moonstone/pull/1604. Please hold off on merging this until the other PR is finalized and merged, since a rebase and update might be required.

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>