Bug 5281947: add to dictionnary option promoted in suggestions.

When several SuggestionSpans are available at a given position, their
content are merged, in creation time order.

As a result, the IME's suggestions are picked before the spell check, and
no add to dictionnary option is created.

This CL modifies the comparator to make easy correction spans appear first
(Voice IME), then misspelled words and then regular suggestions.

Also avoids the creation of a new comparator and length hash map on every display.

Change-Id: I1f9f031a6fdcbbc09f248a192b83051092765f8e