Ensure each character is coverted by at most one LocaleSpan

This is a groundwork to attach LocaleSpan for committed text
in LatinIME.

This CL adds a utility method to ensure that a given range
of the text is coverted by at most one LocaleSpan.  Of course
it could be possible to allow a substring to be coverted by
multiple LocaleSpans at the same time, but ensuring uniqueness
for LocaleSpan is supposed to be a good starting point.

BUG: 16029304
Change-Id: Ic33a7178d0df1f05d3626aeb5773ec902254703f