Clarify when IMMS#mSettings gets updated

As a preparation to introduce per-user InputMethodSettings cache, this
CL aims to clarify when

  InputMethodManagerService#mSettings

gets updated and how, by extracting its update logic out of

  InputMethodManagerService#buildInputMethodListLocked

and renaming that method to

  IMMS#postInputMethodSettingUpdatedLocked.

This is still a mechanical code change. There must be no observable
behavior change.

Bug: 309837937
Test: presubmit
Change-Id: Ie37cd8043f15f2a820205542b4d9d67f1933ca82