Replace some mCurrentUserId with userId in IMMS

This is a follow up CL to my previous CL [1], which hooked up major
per-user call flows in IMMS.

The observable behavior should remain to be the same for single user
mode, and for concurrent multi-user mode the new behavior should be
the right one.

 [1]: I9d4615e6da979204c21555e040a687163c1460ab
      e08e5c4a

Bug: 350386877
Bug: 325515685
Flag: android.view.inputmethod.concurrent_input_methods
Test: presubmit
Change-Id: Iba3504ccfea7477f98b8f9c4dbef2bf5ebf061fb