Simplify IMMS#onImeSwitchButtonClickFromClient()

This CL simplifies

  InputMethodManagerService#onImeSwitchButtonClickFromClient()

which was recently added [1] to as a preparation to support new IME
Language Picker (Bug 311791923), by passing

  InputMethodPrivilegedOperationsImpl#mUserId

instead of propagating UserHandle.myUserId() from the IME process in
favor of simplicity.

Other than the code simplification, there must be no behavior change
in this CL.

 [1]: I633da5b03fa50ec1ccf515a4df5538e397d15442
      61e1f831

Bug: 305849394
Flag: android.view.inputmethod.ime_switcher_revamp
Test: presubmit
Change-Id: Ib21678d864ad2e0a5edf72eefe54d3b6fe4047d4