Replace IMMS#mSettings with IMMS#mCurrentUserId

This CL removes

  InputMethodManagerService#mSettings

as the same instance can be obtained from

  InputMethodSettingsRepository

as long as we keep track of the current user ID.

This is a mechanical refactoring CL. There must be no user observable
behavior change.

Bug: 305849394
Fix: 329163064
Test: presubmit
Change-Id: Idae0fb6d1a9182efe917f81a1d881d488a1b57e5