I need to generate an unsigned, release APK (where someone else will then sign it and release it to the store- with the someone else handling the keys and e
In the release build type, don't specify a signingConfig at all, and your build won't be signed. Release builds don't pick up the default signing config that debug builds get, so it should work for those.
There's a discussion on the adt-dev mailing list about it.
Bear in mind that to build from Android Studio, you'll need to go to the Gradle tasks window and choose the assembleRelease task; normal builds via "Make Project" don't actually build the final APK.