PRODUCT_BUILD_PROP_OVERRIDES support

Summary:
Vital for apps that do hard identity checks like vendor and drm stuff

Change-Id: Ieaed55f62a3d03b7aaf9f0b20857ad8b05f18d6f

Test Plan: test first

Reviewers: synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D376