Cleanup LocaleList docs given API review

-- Remove default constructor from public API since getEmptyLocaleList exists
-- Merge the Locale and Locale[] constructors by providing a single Locale… varargs constructor
-- forLanguageTags, get, toLanguageTags, size, need docs
-- get(int location) should be get(int index)

Plus general docs improvements

Bug: 28296200
Change-Id: I8b4e67184f8c723daebcd251f04947d48bbb5478