Expose FlashPartition members

This change makes the FlashPartition's FlashMemory reference protected,
allowing derived classes to access it. The ParititionPermission member
is also exposed through a writable() function.

Change-Id: Ib0252e4d98863081207d6aeac95d95d6d933dae3
diff --git a/pw_kvs/public/pw_kvs/flash_memory.h b/pw_kvs/public/pw_kvs/flash_memory.h
index d61a896..0460922 100644
--- a/pw_kvs/public/pw_kvs/flash_memory.h
+++ b/pw_kvs/public/pw_kvs/flash_memory.h
@@ -216,8 +216,13 @@
            address;
   }
 
+  bool writable() const {
+    return permission_ == PartitionPermission::kReadAndWrite;
+  }
+
  protected:
   Status CheckBounds(Address address, size_t len) const;
+  FlashMemory& flash() const { return flash_; }
 
  private:
   FlashMemory& flash_;