Remove a special rule from setExplicitlyEnabledInputMethodSubtypes()

With this CL

 InputMethodManager#setExplicitlyEnabledInputMethodSubtypes()

no longer has any special rule on whether the target user is the
current IME user or not.

Overall the observable semantics should remain unchanged.

Bug: 350386877
Test: presubmit
Flag: EXEMPT refactor
Change-Id: I3a963cb580ae1d8c252550d8c9fcfaca5313a8a2