libsnapshot: Add a maximum block count to CowWriter.

Once COWs are allocated, their size is fixed, and we don't want to
write beyond the end. To make this validation less tedious, the virtual
methods of ICowWriter have been made internal. The user-facing API calls
do validation before calling the internal variants.

Bug: 168554689
Test: cow_writer_test
Change-Id: Ic9ebb5bc4b601180d35d915c47cd9c537bc423fa