InputMethodInfo attr to suppress spell checker.

If InputMethodService declares suppressesSpellChecker="true", the system
SpellCheckerService will be disabled.

With TextView, when IMSs are switched while the user is editing,
SuggestionSpans that were previously added will not be removed even if
the next IMS declares suppressesSpellChecker="true".
This is because we don't know if the SuggestionSpans were from Spell
Checker, App, or IMS.

Bug: 153473490
Test: atest CtsInputMethodTestCases:SpellCheckerTest
Change-Id: Ia22b8758111087818beea8c07b1c173f1a94b8e4