Temporarily make binding controller no-op for visible bg users

This is a quick folow up to my previous CL [1], which attempted to
incrementally implement concurrent multi-user IME without any
observable issues.

In case the above change had caused NullPointerException in

  InputMethodManagerService#reRequestCurrentClientSessionLocked()

let's make InputMethodBindingController no-op again by not calling

  InputMethodBindingController#setSelectedMethodId()

for now.

Note that the behavior remains to be fully guarded behind

  IMMS#mExperimentalConcurrentMultiUserModeEnabled.

There must be no observable behavior change unless the above settings
is enabled.

 [1]: I414a228e068fa53223234fe1db3e813474841604
      f7d90f36

Bug: 341558132
Fix: 344883373
Test: presubmit
Flag: android.view.inputmethod.concurrent_input_methods
Change-Id: Ie8cea0888fab987d48410ae7638423f56d7029a2