Merge "libsnapshot: Add CowWriterBase, clean up CowWriter." am: 812aaa9620

Original change: https://android-review.googlesource.com/c/platform/system/core/+/2583491

Change-Id: If51e8c92a73663c7243695083926738dcfb50a96
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>