Implement an experimental init code for visible bg users

This CL implementas an experimental initialization logic for visible
background users in InputMethodManagerService.

When a visible background user becomes available the following fields
will be initialized.

 * Settings.Secure.DEFAULT_INPUT_METHOD
 * InputMethodBindingController#mSelectedMethodId

Keep in mind that this type of initializations are unique to
concurrent multi-user IME support, and we are effectively introducing
a new code fragmentation, which probably needs to be addressed in the
future.

Note also that callbacks from InputMethodBindingController to IMMS are
not yet fully multi-user aware as you can see by searching "mService."
in InputMethodBindingController.java. You cannot yet call

  InputMethodBindingController#bindCurrentMethod()

until these callbacks are fully upgraded to be multi-user aware.

There must be no observable behavior change on non-automotive form
factors. This behavior is fully guarded behind

  IMMS#mExperimentalConcurrentMultiUserModeEnabled.

Fix: 341199701
Flag: android.view.inputmethod.concurrent_input_methods
Test: presubmit
Change-Id: I82cfc366b614d7e55a0f4fa11d5eaf5f0f8eb0a8