Add a check for misspelled span change.

Previously, misspelled span is directly used in AddToDicitonary's
OnClickListener. It can crash because misspelled span can have been
updated since SuggestiosPopup is shown.
Also, AddToDictionary didn't work correctly in full screen extracted
mode. Use findEquivalentSuggestionSpan to resolve it.

Bug: 27557526
Change-Id: I51d713dbdd0c44ea2c553164b81b1123a2325aa1