Support regular fs-verity Merkle tree

In addition to the legacy "apk-verity" Merkle tree that skips the
Signing Block, now the builder supports the regular Merkle tree.

The class and methods are made public because it will be used in
different package.

Test: atest PtsApkVerityTestCases
Test: With other changes, observe the hash matches what other tool
      generates.
Bug: 112037636
Change-Id: I3d96932cd16f4f330170547d6a7689d2997e4b85