Add soong extra config file

Soong extra config file contains Makefile variables that Soong itself
doesn't need to know. It will be read by product_config module to
provide Makefile variables to build tools e.g. build.prop.

Bug: 346214955
Test: see output of product_config module
Change-Id: I045b2722ae85a10d33b9e6b832f4cc2d6789e59a