Create an apex_contributions module type

This module type does not any build actions, but provides metadata about
the contributions of the mainline module to other mainline modules/
platform. One immediate use case for this metadata is source vs
prebuilts selection

Each mainline module can have more than 1 metadata modules. Static build
flags will be used to select the right one in trunk.bzl, next.bzl, ...

Bug: 308174923
Test: go build ./android
Change-Id: I5ead0c38fe64b78de08db1736d97a8ab7374d6d9