libsnapshot: Split CowReader into CowParserV2.

Remove format-specific logic from CowReader and split it out into a new
class called CowParserV2. To make reading the header easier, the
version and size bits are now in a separate CowHeaderPrefix struct.

Bug: 280529365
Test: apply OTA on CF
      inspect_cow
Change-Id: I29b5617ec094d4fb0c284485883d2e921a5bdbf8
11 files changed