Fix confusions around NOT_A_SUBTYPE_ID in IMMS

This is a follow up to my previous CL [1], which aimed to address
confusions around "subtypeId" in InputMethodSettings.

It turns out that there remains one more instance in IMMS. Let's fix
it in the same way.

There must be no observable behavior change as both constants use the
same magic number "-1" at this moment.

 [1]: Ic4968449ea89debc3bc9e519a724c16222851235
      e95076af

Bug: 346658341
Bug: 346662143
Bug: 347093491
Test: presubmit
Flag: EXEMPT refactor
Change-Id: I35fbf5afccc30b2dc876fb23a3eda9bdf5effc8a