Use LocaleList in KeyboardLayout.

As an exercise for a new API candidate LocaleList class, this CL does a
mechanical refactoring to replace Locale[] with LocaleList in
KeyboardLayout class.  Note that what changed in this CL is just an
implementation details that is never exposed to application developers.

One take-away from this exercise is that finding the best-match locale from
an ordered locale list is really a common pattern.  Perhaps we may want
to have a guideline for this kind of situation.

Change-Id: I142379afbaf24d524ff09cf6c7ee7720150f7489