Add Executor param to TSM#newSpellCheckerSession().

SpellCheckerSessionListener callback methods will be called on
the provided executor.

This CL also adds more description for 'bundle' parameter.

Bug: 179804789
Test: atest CtsInputMethodTestCases:SpellCheckerTest
Change-Id: I43f9f04fc335ad45d2580f4711b19aee214bc7dd