Resolve conflict AVB rollback index location

Add an `--avb-resolve-rollback-index-location-conflict`
option in merge_target_files. When this option is set,
the merge tool will resolve conflicting index locations
by assigning the smallest unused index location.

This is to support merging system and vendor target files
from two different targets. In this case, the two target
files may have conflict rollback index location because
they were built independently.

Test: atest releasetools_test
Test: validate_target_files *-target_files-*.zip
Test: merge_target_files &&
        add_img_to_target_files &&
        img_from_target_files &&
        flash device
Bug: 300604688
Change-Id: Ibd18ef2a9f3784157fe17966f5364c3c81c9bd9f