Introduce a mechanism to hide the indicator speculatively

This is an optional optimization to reduce the UI latency.

Imagine that the commit indicator is now displayed and the
composing text is being updated, it is highly likely that
the commit indicator will disappear unless the application
rejects the setComposingText request.

If we assume that the application will accept the new
composing text without any modifications, we can hide the
indicator without waiting for the arrival of new
CursorAnchorInfo event.

This optimization isn't dangerous because we can show the
indicator again when we receive new CursorAnchorInfo event
and the assumption is turned out to be invalid.

Change-Id: Id59c6607a1029782410611e768791af9984f14ac