The new command is part of the transfer.list and allows us to compute the hash
tree on non-ab devices.
The required arguments for the hash_tree computation are:
hash_tree_ranges
source_ranges
hash_algorithm
salt_hex
root_hash
Bug: 25170618
Test: unit tests pass; run simulator with compute_hash_tree
Change-Id: I8ff0d582cc8adabb8a060db7845f38b35b28e62c