Unify user init logic in IMMS#onUnlockUser()

With this CL

  InputMethodManagerService#onUnlockUser()

starts using a unified logic for concurrent multi-user mode instead of
relying on

  InputMethodManagerService#initializeVisibleBackgroundUserLocked().

There must be no behavior change in single-user mode.

Bug: 353751919
Test: presubmit
Flag: android.view.inputmethod.concurrent_input_methods
Change-Id: I174d264d94d8e279c9a2f16f3167794a09f2f5ea