Handle IMMS#onActionLocaleChanged() on the IO thread

With this CL

  InputMethodManagerService#onActionLocaleChanged()

starts being executed on the IO handler thread rather than the general
handler thread (a.k.a. IMMS UI thread), as it queries available IMEs
via PackageManager APIs, which may a bit slow.

Semantically there should be no observable behavior in this change.

Bug: 343601565
Test: manually verified by changing the device locale
Flag: EXEMPT refactor
Change-Id: I66ee52db627f92a466b4e067c9b3dc114150c6db