Remove a special rule from setAdditionalInputMethodSubtypes()

With this CL

 InputMethodManager#setAdditionalInputMethodSubtypes()

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: Ica0c52ec872066494e5db351b1dbd1fca0e3b1d4