libsnapshot: Bump up the kCowVersionManifest version

kCowVersionManifest will be 2. This should now
be in sync with kCowVersionMajor.

Bug: 183863613
Test: OTA with new COW format (by enabling scratch space option)
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: Ia6c31e399de723ee83459b59d6b076b48f5c88d5