Fix minor missing/misused annotation in IMMS

This CL is to make it clear on what remaining fields need to be
updated for concurrent multi-user IME support.

This is only about source level annotations. There must be no change
in the final artifacts.

Bug: 305849394
Test: presubmit
Change-Id: Ib59af584607ab534758d57af43c8b45c4ee9780f