vendor apex: label apex_manifest as vendor_apex_metadata_file

Since vendor APEXes are installed in vendor partition, there shouldn't
be system_file according to Treble. Instead, we can force-label / and
/apex_manifest.pb as vendor_apex_metadata_file so that apexd (and other
system components) can still read them.

Bug: 285075529
Test: m nothing (soong test)
Change-Id: Idb36b8c4c68b29e2235dbda38ee323d4b781e1d5