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_interface.h b/payload_consumer/verity_writer_interface.h
index db7988e..37ed605 100644
--- a/payload_consumer/verity_writer_interface.h
+++ b/payload_consumer/verity_writer_interface.h
@@ -31,9 +31,6 @@
  public:
   virtual ~VerityWriterInterface() = default;
 
-  virtual bool Init(const InstallPlan::Partition& partition,
-                    FileDescriptorPtr read_fd,
-                    FileDescriptorPtr write_fd) = 0;
   virtual bool Init(const InstallPlan::Partition& partition) = 0;
   // Update partition data at [offset : offset + size) stored in |buffer|.
   // Data not in |hash_tree_data_extent| or |fec_data_extent| is ignored.
@@ -41,6 +38,10 @@
   // blocks has passed.
   virtual bool Update(uint64_t offset, const uint8_t* buffer, size_t size) = 0;
 
+  // Write hash tree && FEC data to underlying fd, if they are present
+  virtual bool Finalize(FileDescriptorPtr read_fd,
+                        FileDescriptorPtr write_fd) = 0;
+
  protected:
   VerityWriterInterface() = default;