libsnapshot: Changes to AddCopy() API

If the copy blocks are contiguous, add a third
argument which takes the number of blocks
which are contiguous. With this, update engine
can call the API in one shot for all the
contiguous COPY operations.

This is required for batching the I/O
for async writes.

This should still continue to support the existing
API where we pass one COPY block at a time.

Bug: 254188450
Test: Incremental OTA from A->B with new API changes in A
      Incremental OTA from A->B with plain VAB
      cow_api_test
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I7edc52a152e02de28a44ef1dc2c88b76a28c4109