Preserve spans when splitting sentences

This CL fixes a bug that span information was unintentionally
removed when given sentences are split into words. Nothing is
changed in KitKat and prior because TextInfo never returns
spanned text until L.

Note that the spell checker service still doesn't rely on
span information even with this CL.

BUG: 16419551
Change-Id: I6ffb75ee6add4ae20e9d3dcfb90b65f097d70fba