Refactor verity reads/writes to a separate fucntion

Test: th & serve an OTA with veity enabled, vabc disabled
Change-Id: Ib1d5549ac615504a47c96a12b046975cfff01886
diff --git a/payload_consumer/verity_writer_stub.h b/payload_consumer/verity_writer_stub.h
index f8d68ca..a20db03 100644
--- a/payload_consumer/verity_writer_stub.h
+++ b/payload_consumer/verity_writer_stub.h
@@ -26,10 +26,9 @@
   VerityWriterStub() = default;
   ~VerityWriterStub() override = default;
 
-  bool Init(const InstallPlan::Partition& partition,
-            FileDescriptorPtr read_fd,
-            FileDescriptorPtr write_fd) override;
+  bool Init(const InstallPlan::Partition& partition) override;
   bool Update(uint64_t offset, const uint8_t* buffer, size_t size) override;
+  bool Finalize(FileDescriptorPtr read_fd, FileDescriptorPtr write_fd) override;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(VerityWriterStub);