Add FileDescriptorPtr interface to verity writer

In VABC, read/write of target partition is no longer done via unix file
descriptor. Instead it's done via CowWriter and FiledescriptorPtr.
We need abstraction for verity writer.

Test: treehugger

Change-Id: Id638b4e5a2cea4ab97927a6e7089170a3e257dee
diff --git a/payload_consumer/verity_writer_stub.h b/payload_consumer/verity_writer_stub.h
index ea5e574..f8d68ca 100644
--- a/payload_consumer/verity_writer_stub.h
+++ b/payload_consumer/verity_writer_stub.h
@@ -26,7 +26,9 @@
   VerityWriterStub() = default;
   ~VerityWriterStub() override = default;
 
-  bool Init(const InstallPlan::Partition& partition) override;
+  bool Init(const InstallPlan::Partition& partition,
+            FileDescriptorPtr read_fd,
+            FileDescriptorPtr write_fd) override;
   bool Update(uint64_t offset, const uint8_t* buffer, size_t size) override;
 
  private: