Simplify InputMethodUtils a bit

This is a follow up to our previous CL [1], which introduced

  InputMethodManager#getCurrentInputMethodInfo().

As a preparatoin to start maintaining multiple instances of
InputMethodSettings at the same time, this CL simplifies
InputMethodUtils a bit without changing the behavior.

 [1]: I60a0f67bf7d261d3a4a733adcb8a022ceac6e1db
      2422bcff

Bug: 255799034
Bug: 309837937
Test: atest CtsInputMethodServiceHostTestCases:MultiUserTest
Change-Id: I1a9a09655bd63fc5d8cd1811f82c04001d21c22d