Optimizations and safeguards.

- Don't call finishComposingText when useless.
- Add safeguards against calling setComposingRegion when the
  data returned by the editor is inconsistent.
- Cancel pending recorrection messages when new messages arrive.

Bug: 8842941
Bug: 8845001
Change-Id: I939701033cf5c2bbd85871ecf83e329021ddeb91