Too many SpellCheckSpans are created.

Removed the Runnable in SpellChecker, spell check is triggered at the end
of updateSpellCheckSpans instead of when a new SpellCheckSpan is created.

Cache the spans in updateSpellCheckSpans to limit the calls to getSpans.

When typing, every new letter in a word will create a SpellCheckSpan
(this is needed in case the user taps somewhere else on the screen)
The SpellCheckSpans are pooled in SpellChecker to limit unnecessary new
SpellCheckSpan creation.

Minor optimization on test order in getSpans to avoid some calculation.

Spell check is not started everytime the selection is changed (would be
triggered when the insertion handle is moved). Explicitely do that only
on tap.

Change-Id: Ibacf80dd4ba098494e0b5ba0e58a362782fc8f71