Fix mainline_mixed build

build_mixed script can't merge system and product VINTF
data yet.

Fixes: 131418170
Bug: 131425279
Test: build 'target_files_package' for 'mainline_system_google_arm64',
see META/system_manifest.xml

Change-Id: I366d9bc802ee0e6bdf8fe480303f3fee827c579d