Regard out/soong/release-config as a source directory

Files in out/soong/release-config are generated from lunch or kati
configuration phase. They don't have rules in ninja, but always
provided as source files.

Bug: 324996303
Test: m checkbuild
Change-Id: Ibfb76a8c58a81a4d768b29d80151fa47e2ebb9c1