Introduce IMMS#mExperimentalConcurrentMultiUserModeEnabled

This is a preparation to start implementing a special runtime mode in
InputMethodManagerService that is used when and only when concurrent
multi-user IME support is enabled.

This CL only introduces a boolean field to IMMS without any behavior
change.

Bug: 341199701
Test: presubmit
Flag: android.view.inputmethod.concurrent_input_methods
Change-Id: If8f48331256e28cf34794043903cc7cf619f2aed