Allow products to define their own hidden api stubs

This enables other verticals/products to expand the list of APIs that
apps can call on those devices.

Bug: 112190341
Test: local test with app
Merged-In: I4dc983273a1beb83d0b81ab026945880a10bbd8c
Change-Id: I4dc983273a1beb83d0b81ab026945880a10bbd8c
(cherry picked from commit f9bf755d)