Add comments for some int fields in EditorInfo

As a preparation to add a new flag into EditorInfo, this CL aims to make
it clear what bits in EditorInfo are already used and what bits are
still available for new features.

There should be no behavior change because there is no code change.

Bug: 28157942
Test: compile, as there is no actual code change
Change-Id: Ie4b216dbb0978d8dc1c5e91762b87d065c7a1cac