Take apart LocalePicker#onListItemClick() into two

It takes some time when Locale is actually switched by the system,
which looks "system halt". Previous implementation doesn't allow
developers to cope with the situation and they just had to let
users wait for a moment.

This change allows the LocalePicker users (developers) to hook
the item change and delay the actual halt. If they don't want it,
calling updateLocale() immediately works fine.

Related
- I29685118467bd2a68b80abcf234f91098839be0b
- Iba7378e51e6d5318e901045ebeda63ea954a0647

Change-Id: I4062a70e11600d87e889b40d30796387c7069e21