BHV-4060: Only reflow when necessary (initial expansion, picker property change).

Issue
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A reflow is 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.

There would be an issue if the property names did not match between `moon.ExpandableIntegerPicker` and `moon.SimpleIntegerPicker`, so maybe the programmatic instantiation of the observer is not really necessary...

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>