Enable StringUtils to split CharSequence like String#split

This is a groundwork for enabling LocaleSpan.

To enable LocaleSpan everywhere, we need to switch to
CharSequence from String so that Span infromation can be
preserved end-to-end.  To achieve this, we need to have
CharSequence version of String#split.

BUG: 16029304
Change-Id: I0dd103185dcf62fb1e25054a374340790e6a4678