Picker - determine initial scroll position for variable height items

[bmingles]

• The Picker currently calculates the initial scroll position for the selected item by finding the index and multiplying by item height. This only works for items with consistent heights. Spectrum exposes the keys as data-key attributes, so we should be able to find the target position without knowing the item height.
• Also, there is a subtle bug when using uncontrolled Pickers. Since the position is determined by selectedKey ?? defaultSelectedKey, the position will be incorrect if the user selects a new item and then re-opens. We'll need to maintain the selection internally.