Separate apex extraction from the ApexSet

This follows the same pattern as is used by the prebuilt_apex in order
to make files encapsulated with a prebuilt .apex file available for use
by the rest of the build. It separates the extraction of the
appropriate apex from the zip supplied to apex_set into a separate
module type whose output is used by the apex_set and will (in a follow
up change) be used by the deapexer module too.

Bug: 181267622
Test: m droid
Change-Id: Icd13ce1f21845494e875d3e092abd4a9d2efca57