libsnapshot: Refactor code

Move all files related to libsnapshot_cow library to
a seperate directory. Libsnapshot directory is getting
crowded and we will be adding more files to this library
with async writes.

No changes to any logic.

Bug: 254188450
Test: Full OTA on Pixel
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: If84ff730d477c85a8ef57864d2185f2f897cf8e0