Simplify IMMS#updateSystemUiLocked()

With my previous CL [1], it is now obvious that
InputMethodManagerService#updateSystemUiLocked() is always called with
InputMethodManagerService#mCurToken.  With that, this CL mechanically
simplifies InputMethodManagerService#updateSystemUiLocked().

There should be no behavior change.

 [1]: I086dc0381078fc292db8132bc64d920ef7646c1c
      6fee8818

Bug: 34851776
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Change-Id: I86cae337ec3f982774a626f996db1b1d62cc529f