Make sure to not block SystemService#onUserStarting() in IMMS

As part of our on-going effort to make sure that slow I/O operations
will not block the system in InputMethodManagerService, we start
dispatching

  InputMethodManagerService#initializeVisibleBackgroundUserLocked()

into an I/O thread.

The observable semantics should remain unchanged in general.

Bug: 343601565
Test: presubmit
Flag: EXEMPT refactor
Change-Id: Iee0b1c7276f88e4b179b0eb8fb978503bc45760c