Fix digest algorithm

The previous code used Checksum's algorithm definition, which is
obviously inconsistent with the enum in our atom. It's a bad idea to use
Checksum, so replace it with a new private data class, Digest.

Fixes: 271151129
Test: Inspect the atom, the value looks correct (I'm seeing
      CHUNKED_SHA512 for APEXes and CHUNKED_SHA256 for MBA)
Change-Id: Ib5ef1f7cdf53207f42651c572a5566f8c154d7c6