Refine binding fields in InputMethodManager (1/N)

-. Introduce BindState class to manage input method
   binding related fields.
-. Migrate mIsInputMethodSuppressingSpellChecker flag
   and IInputMethodSessionInvoker into BindState.

This is a mechanical refactor which should no user
visible change.

Bug: 236937383
Test: atest CtsInputMethodTestCases
Change-Id: I010bc0ad196492bfac244b756dcfd986b9075698