libsnapshot: Remove dependence between CowWriterV2 and CowReader.

The plan of record for the new CowOperation layout is to automatically
translate from CowOperationV2. However, CowWriterV2 currently imports
directly from CowReader, which means we'd need two translation steps.

Luckily we now have CowParserV2, which means we can directly import into
CowWriterV2.

Bug: 280529365
Test: vts_libsnapshot_test
Test: cow_api_test
Change-Id: I11863dcb483f0448686d6c492d8eb128840ce18c