libsnapshot: snapuserd: Enable read-ahead functionality

This enables read-ahead functionality by having
scratch space in the COW

Bug: 183863613
Test: OTA tests with new COW format

Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I7988687c81d0ea239e71695818199db4653ddb80