Label /vendor_file_contexts as file_contexts_file

vendor_init doesn't have permissions to read rootfs labeled files, but
needs to read /vendor_file_contexts to do restorecon correctly.  This
file is a file_contexts file, so labeling it as such seems appropriate.

Test: bullhead + vendor_init doesn't hit this audit
Change-Id: I475e9735616c2426b9c7073700272f878ced2135