Use IO thread to handle IMMS user switching

This is a preparation before enabling strict mode check on IO
operation on the handler thread (UI thread) in IMMS.

As other user-related operations are also offloaded to IO thread,
moving user swithcing task to the IO thread would also give us a
strong order garantee across user-related operations (e.g. user
addition/removal, unlocking, and so on).

Observable semantics should remain unchanged.

Bug: 196206770
Bug: 343601565
Test: presubmit
Flag: EXEMPT refactor
Change-Id: I3b6212d724b1d3bec89aa7fd7f0bc12187376719