Editor uses a SpanWatcher to track EasyEditSpans

Will also fix Bug 6344997

The previous TextWatcher mechanism was inneficient. It require an
expensive getSpans() call to retrieve all the spans and then search
for the one we're interested in in case it has been changed.

The SpanWatcher is faster, it will broadcast the add/changed/removed
events we're interested in.

Now that we can rely on SpanWatcher, use it to directly track
addition and removals of EasyEditSpans.

No unit test for this feature which require an integration with
the voice IME. Easy to test manually though.

Change-Id: Idabcacc48c479bf9868d5204c0b0ca709207ede2