Preserving /avb/* keys to /metadata

Those keys will be used for key revocation check by DSU installation
service. Note that failing to copy those keys to /metadata is NOT fatal,
because it is auxiliary to perform public key matching prior to booting
into DSU images on next boot. The actual key matching will still be done
on next DSU boot.

Bug: 146910547
Test: boot device, checks the avb keys are copied to /metadata/gsi/dsu/avb/.
Change-Id: I25a4eba82e84288bac7a859205c920628a063651