Here's a document which describes the icons we need:
http://tools.android.com/contributing/iconwishlist
As you can see there are a LOT of icons but we don't need to replace
them all at once - gradually improving the look is fine (and we
recently got new launcher icons for all the tools for example).
For development of the Android tooling itself (not for Android
platform or how to build apps -using- the tools) we have a dedicated
forum:
http://groups.google.com/group/adt-dev
That might be a great place to discuss specific icons in ADT more.
For the actual contribution you'll still need to go through uploading
the files via gerrit, after agreeing to the contributor license:
http://source.android.com/source/licenses.html
-- Tor