Accessor for FlashPartition::start_sector_index_

What it says on the tin. Required by derived classes overriding
PartitionToFlashAddress.

Change-Id: I643ce12174856d2e4a4b24bcc9124378398c7b13
diff --git a/pw_kvs/public/pw_kvs/flash_memory.h b/pw_kvs/public/pw_kvs/flash_memory.h
index b5b9716..0ce3378 100644
--- a/pw_kvs/public/pw_kvs/flash_memory.h
+++ b/pw_kvs/public/pw_kvs/flash_memory.h
@@ -230,6 +230,8 @@
     return permission_ == PartitionPermission::kReadAndWrite;
   }
 
+  uint32_t start_sector_index() const { return start_sector_index_; }
+
  protected:
   Status CheckBounds(Address address, size_t len) const;
   FlashMemory& flash() const { return flash_; }