libsnapshot: Fix footer values and verification

A few values in the footer were being set inconsistently. We weren't
verifying them, so it didn't matter. This adds verification and sets
them properly.

Change-Id: I669a03a6e1e87ace31775aba5b67dde9b6e5ecf5
Bug: 168829493
Test: cow_api_test
2 files changed