Verity writes propogate progress report

Test: tested on cuttlefish device with inc_ota from build 9263663 9419942. Ensured progress grows linearly from 0 - 0.6. Half time spent in read step, half in Verity Writes.
Change-Id: Iffa16571e828909cd6ac58f262736c0b6ca7b066
diff --git a/payload_consumer/verity_writer_interface.h b/payload_consumer/verity_writer_interface.h
index 8b4f080..3ebe768 100644
--- a/payload_consumer/verity_writer_interface.h
+++ b/payload_consumer/verity_writer_interface.h
@@ -55,6 +55,9 @@
   // Returns true once FEC data is finished writing
   virtual bool FECFinished() const = 0;
 
+  // Gets progress report on FEC write
+  virtual double GetProgress() = 0;
+
  protected:
   VerityWriterInterface() = default;