This CL simplifies the wrapper method for
TextInfo#getCharSequence() because in the almost all cases we want
to use the result of TextInfo#getText() as the default value.

BUG: 16029304
Change-Id: I62f987aed8ca42b093255e6c0703d6120fa01299

This is a groundwork for enabling LocaleSpan support in spell
checker service. TextInfo#getCharSequence is supposed to be
available in API Level 21 and later. We need some abstraction
layer as usual.

BUG: 16029304
Change-Id: Iafbf2877db393dbd040295c21d429334e3a13a5f