Make LocaleList constructor non-nullable

This commit makes the LocaleList constructor require non-null
arguments in all cases, and fixes all uses of LocaleList that could
previously pass a null to use getEmptyLocaleList() instead (which is
preferred anyway becaues it avoids an allocation.

Bug: 28460668
Change-Id: I4b8b3cfa82914412731c2b79003951c46cb2afa1