Clarify how IME ID is treated in ImeSubtypeListItem#compareTo()

This is a preparation to remove dependency on TreeMap from
InputMethodAndSubtypeList#getSortedInputMethodAndSubtypeList().

This CL itself must not have any behavior change.

Bug: 64612471
Test: atest InputMethodSubtypeSwitchingControllerTest
Change-Id: I5684bbbb3b223c4e6cc7892b2620fb91356b7037