Sign system_other.img with AVB

Support signing system_other.img but shouldn't include it into the
top-level vbmeta.img. system_other verifiation will not be included
in /vbmeta chains and will be done separately.

Bug: 112103720
Test: avbtool info_image --image $OUT/system_other.img
Test: avbtool info_image --image $OUT/vbmeta.img, checks 'system_other' is NOT included.
Test: Checks $OUT/obj/PACKAGING/system_other_intermediates/system_other_image_info.txt
      See the following:
        avb_system_other_hashtree_enable=true
        avb_system_other_add_hashtree_footer_args=--rollback_index 1551744000
        avb_system_other_key_path=external/avb/test/data/testkey_rsa4096.pem
        avb_system_other_algorithm=SHA256_RSA4096

Change-Id: Ia152aaab1387dcf556a42222adb39ea76881263a