fs_mgr_avb_ops: stores the by-name prefix of /misc instead of fstab struct

fs_mgr_avb_ops->read_from_partition() relies on the by-name prefix to
get the device file when reading a partition. Previously we store the
fstab struct and use fs_mgr_get_entry_for_mount_point() for every read.

It's better to do that once and store the by-name prefix for later use.

Bug: 33254008
Test: test AVB on bullhead
Change-Id: Ie999851b529f984e29ccbf39fb52a8220056fe0c