Introduce per-user InputMethodSettings cache

As a preparation for concurrent multi-user support in
InputMethodManagerService, this CL introduces

  InputMethodSettingsRepository

as a per-user InputMethodSettings repository.

Note that InputMethodSettingsRepository.get(userId) has not yet used
in this CL thus there must be no user observable behavior change.

Bug: 309837937
Test: presubmit
Change-Id: Iccede363caffb04087f511b6585cf07565dafda1