Do not trigger IME user switching in experimental multi-user mode

In the (experimental) concurrent multi-user mode, all the active IME
users should be treated equally and there should be no current IME
user any more.

This CL makes it clear that

 * Lifecycle#onUserSwitching can be ignored
 * No need to change the "current IME user" from
   IMMS#startInputOrWindowGainedFocus

Note that the semantics on work-profile when concurrent multi-user
mode is not yet defined, as both personal and work profiles are
supposed to share the same display.

The above special behaviors are guarded behind a runtime flag

  android.view.inputmethod.concurrent_input_methods.

There must be no behavior change unless it is enabled (and other
conditions defined in
IMMS#shouldEnableExperimentalConcurrentMultiUserMode() are met).

Bug: 341558132
Test: presubmit
Flag: android.view.inputmethod.concurrent_input_methods
Change-Id: I48205f8cbd5db77b46bdfd8af19b306c6150e352