Send TYPE_VIEW_TEXT_CHANGED a11y event when SuggestionSpan is added

When user types space after a word, we receive a text
change event, but the suggestion span is not added to
the text yet. The spell checker adds the span after the
text change event is sent. Previously we send the event
in onSpanAdded in TextView, but we don't do anything or
update the before text for span removed. This is a bit
confusing and error prone. This change moves the send
event logic into spell checker.

Bug: b/143378480

Test: tested with talkback.
Change-Id: Ibd45843494304602b177df8da520a51058989f10