Enable CursorAnchorInfo to contain composing string

This CL adds one more functionality to CursorAnchorInfo
that enables applications to associate the composition
string with its positional information. This is useful
for an IME to handle CursorAnchorInfo asynchronously.

This is also useful for the framework to detect if
the application is unnecessarily calling
IMM#updateCursorAnchroInfo with duplicate event.

BUG: 14579622
Change-Id: Ie75c17b523dad33e97b08c15f5f5267573ce2063