Move #mSwitchingController to UserData

With this CL

  InputMethodManagerService#mSwitchingController

to UserData so that each user can keep maintaining its own instance.

The observable behavior should remain to be the same for single user
mode. For experimental concurrent multi-user mode, the controller is
now properly initialized in

  IMMS#experimentalInitializeVisibleBackgroundUserLocked().

Bug: 305849394
Test: atest FrameworksInputMethodSystemServerTests
Flag: android.view.inputmethod.concurrent_input_methods
Change-Id: I47c641623aebc6885fa005f4ff688c3786c64e8d