Bug 5384674: When only one suggestion is returned, it is displayed twice

Two separate issues here:
- The results of the spell checker may be identical to the one set by the IME. Since
we merge the spans, the entries are duplicated. Filter spell checker results to avoid
these duplicates.

- When the text is saved on rotation, the spans are saved and restored. Since we start
a new spell check when the window is attached, it also doubles the size.

Change-Id: I21e1a5ae1b264bc97f44d762e4589bf520c6c19c