Clean up prebuilt stub deps & add comment

The non-updatable stub depends on a prebuilt platform stub, so that it
can reference APIs defined in modules. Clean up the list of previously
needed prebuilts that are no longer necessary, and add a comment about
when it's suitable to add new prebuilts to this list.

Test: m
Change-Id: Idd5f57c774db4fc48abe4bab87c1b393079cae4d