VSDK: etc snapshot may have sub_dir for relative_install_path

prebuilt_etc may use sub_dir property to define a relative install
path. Use SubDir() to read either sub_dir or relative_install_path
property.

Bug: 282080219
Test: m vendor-image
Change-Id: I07a21c5aeb87b403854ed32976d26e70b046b8c1