Simplify InputMethodSettings#isCurrentProfile()

This CL simplifies

  InputMethodSettings#isCurrentProfile()

without changing the observable behavior.

Given that

  UserManagerInternal#getProfileIds(int userId, boolean enabledOnly)

returns an array that includes "userId" itself, checking userId with

  InputMethodSettings#mCurrentUserId

is considered to be redundant.

This is a preparation to decouple InputMethodSettings from
InputMethodManagerService global instance.

Bug: 309837937
Test: presubmit
Change-Id: Iebe56c2da70c95e584be582a706716c137eeb15b