libsnapshot: Fix some minor formatting bugs

A few minor issues snuck through code review.
Removed a debug line in cow_api_test to persist file.
Removed unused function declaration for cow writer
Switched PLOG to LOG where no errno exists.

Change-Id: Ibb774d6de518fc2a8746e4b9eefc3655c9973c30
Bug: 168554689
Test: cow_api_test