Is it a good idea to include "research interests" section in CV for PhD admissions? If so, what to include in it?
The section on research interests should help to identify these quickly. In principle one could deduce your interests from your publications, talks, thesis and so on, but an exposed lists makes life easier.

However, you can not just tailor your list of research interests arbitrarily to the position as they should be in line with the rest of the CV. If you state that something is one of your top interests but this topic does not appear anywhere else in your CV this looks suspicious. My advice is, to be totally honest here. Do not include anything which is not truly your research interest, and if you include something that does not show up in your CV, have some explanation for this somewhere else (e.g. in a research statement or cover letter). Don't write one of these applications where everything looks like a perfect fit for the position, but nothing is backed up with evidence.