Use IO thread more in IMMS

This CL fixes the remaing cases when the following classes do I/O
operations not in the dedicated I/O thread.

  * AdditionalSubtypeMapRepository
  * InputMethodSettingsRepository

Other than that there must be no observable behavior change.

Bug: 343601565
Test: atest FrameworksInputMethodSystemServerTests
Flag: EXEMPT refactor
Change-Id: Id797c68e7966c9fb3b298a9a0843b942b8ffe004