Track the AVB property name change.

system_avb_hashtree_enable has been renamed to
avb_system_hashtree_enable in commit
3e599ead. 'care_map.txt' is missing due
to the change.

Bug: 63142730
Test: `m dist` and check IMAGES/care_map.txt exists in target_files zip.
Change-Id: I60c269b41df844b50353d357bf67c20f15548642
(cherry picked from commit 3f72176d)