first_stage_mount: reading all avb_keys before chroot

Avb keys used to verify a partition are stored in the first-stage
ramdisk. However, after /system is mounted, init will chroot into it.
This makes those keys inaccessible for later mounts, e.g., /vendor or
/product.

This change retains avb keys by reading all of them before chroot
into /system.

Note that it is intentional to perform public matching for both
preload_avb_key_blobs and fstab_entry.avb_keys in libfs_avb.
As some keys might only be availble before init chroots into /system,
e.g., /avb/key1 in the first-stage ramdisk, while other keys might
only be available after the chroot, e.g., /system/etc/avb/key2.

Bug: 147585411
Test: specify avb_keys for a partition and checks the keys are preloaded
Test: atest libfs_avb_test
Test: atest libfs_avb_internal_test
Change-Id: I6bd490c4215480db2937cdfc3fea0d616e224a91