libsnapshot: Clone worker readers from snapuserd

We'll need to have the Cow Files parsed to deal with xor ops, since
their data location is implicit from the format. Since the relevant data
is all stored in shared pointers, we can pass that data into the workers
without needing to reparse or copy.

Bug: 177104308
Test: builds
Change-Id: I96ac3da1ae620be48e5340c9f146c523b3ce74b6