libfs_avb: support key rotation for standalone partitions

The FstabEntry.avb_key is renamed to FstabEntry.avb_keys, to
allow specifying multiple avb keys, separated by ':'
(because ',' is already used by fstab parsing).

Bug: 124013032
Test: boot live GSI with multiple allowed AVB keys
Change-Id: Iacd3472a1d5a659dfecf09ea6074d622658f4d0b
9 files changed