Add a SpanWatcher to catch programmatic selection changes

When the selection is set via Selection#setSelection, the
Editor needs to know so that it can call back to a bound
IME, if any. This adds a watcher to catch these events.
This is step 2 of a four-step refactoring.

Bug: 8000119
Change-Id: Ia01aee853d5cafb4820fd234bc24b587ad3f7dd0