libsnapshot: Use the COW size from the update manifest.

When Virtual A/B Compression is enabled, the manifest contains the
predicted COW size. Use this instead of the algorithm based on the
kernel COW format.

Bug: 168554689
Test: vts_libsnapshot_test
Change-Id: I545679b4834957ff80a930d91cb44afbadebb66c
9 files changed