Use CharSequence for spell checker to keep spans preserved

This is a ground work to take per word locale information into
consideration in the spell checker. This CL is supposed to change
no user visible behavior.

With this CL, the spell checker session is able to read span
information if necessary.

BUG: 16029304
Change-Id: Icb1e1ecdf40fe0445e14565b685b1b878b746210