Add ApkVerityBuilder to contruct the verity tree

Test: Locally add some code in PackageManagerService to generate the
      verity tree.  Root hash and the tree is consistent with the output
      of apksig.
Test: With local mod, with apk size of 400/100/20/5 MB, verification
      time is about the same for the existing algorithm before and after
      the refactoring.
Test: With local mod, with a 400 MB apk, verification time of the new
      algorithm is slower (2s) than the 1MB-based algorithm (600ms).
Bug: 30972906

Change-Id: Ie429cf9b80884e56a8e0882e1c125c8a3f8feab4