libsnapshot: COW size is computed within the container capability

Enforce the checking of what chunk/sector/byte is written to the COW
size calculator to avoid possible overflows in the container.
If an undesired behavior occurs, the COW computation is aborted and,
the class user will be notified with an empty returned value.

Bug: 176972301
Test: vts_libsnapshot_test
Change-Id: I29f7909780853434a09032c27c943f505c6d0d19