InputConnection is warned when finished

As said in https://android-git.corp.google.com/g/#/c/155992
finishComposingText is indeed too broad of a method.

Introducing a new dedicated method to warn the InputConnection.

Should solve the problems with a negative counter value.

Change-Id: I5525d776916f0c42d5e6d4a4282aed590d7f0e9a