fs_mgr: support 'check_at_most_once' for dm-verity

Parse the hashtree descriptor of the AVB-protected partitions
and add 'check_at_most_once' option to dm-verity table.

Test: build & check dm-verity table
Test: atest --host libfs_avb_test
Test: atest --host libfs_avb_internal_test -- --test-arg \
      com.android.tradefed.testtype.HostGTest:native-test-timeout:5m

Signed-off-by: JeongHyeon Lee <jhs2.lee@samsung.com>
Change-Id: I226a545aa95e07ebd7096dde18c357ac1ed1241c