Add an updater function to compute hash tree

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