Calculate verity hash tree.

Calculate verity hash tree in FilesystemVerifierAction based on configs
specified in protobuf, and write it to target partition before reading
from those blocks.

A new error code kVerityCalculationError was added to report error if
it fails.

Bug: 28171891
Test: update_engine_unittests

Change-Id: I492885a0655bf51043902f578720fffd87e6a3cf