Add comments about TODO Bug 347083680

There have been known questionable behaviors in

  InputMethodManagerService#getCurrentInputMethodSubtypeLocked(),

where 1) its non-null return value is not guaranteed to belong to the
current IME and 2) it can internally update IMMS#mCurrentUserId.

While we don't want to introduce any behavior change at this moment,
let's leave comments for future readers for now.

There must be no side effect in this change.

Bug: 347083680
Test: presubmit
Flag: EXEMPT refactor
Change-Id: I37dbfec0322dbf9d0cad3761b61dc4d07adf23bf