BHV-4060: Only reflow when necessary (first opening, picker property change).

Issue
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A reflow would be run everytime the picker is opened.

Fix
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Instead of defining the observer declaratively in `moon.ExpandableIntegerPicker`, I defined it programmatically to keep it in sync with the `triggerRebuild` observable of `moon.SimpleIntegerPicker`. Would like some extra review on this in case this is not kosher or has some subtle side effects. Now we only reflow the picker upon initial opening, or when one of the properties that triggers a rebuild has changed.

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>