Support getting public key data

This allows the client to get the public key data without need to
provide the expected key data to load vbmeta. Then do the comparison by
themselves, to allow "key rotation".

Bug: 65470881
Test: atest libfs_avb_test
Test: atest libfs_avb_internal_test
Test: boot a device
Change-Id: Icd4e317c2f79cd35e46cdd14f858575ee692facd