[REFACTOR] DynamicPartitionControl: Add GetSuperPartitionName

fs_mgr_get_super_partition_name() should be mocked because
it is an external dependency to libfs_mgr.

In tests, deliberately make GetSuperDevice() to return "fake_super"
instead of "super" to make sure it is mocked properly.

Test: run unittests
Test: manually apply OTA

Change-Id: I0f05d99bf168b6e658052b4bd67dc1e82ab36471
diff --git a/dynamic_partition_control_interface.h b/dynamic_partition_control_interface.h
index 32fbbe4..b3ce4ea 100644
--- a/dynamic_partition_control_interface.h
+++ b/dynamic_partition_control_interface.h
@@ -95,6 +95,10 @@
 
   // Return a possible location for devices listed by name.
   virtual bool GetDeviceDir(std::string* path) = 0;
+
+  // Return the name of the super partition (which stores super partition
+  // metadata) for a given slot.
+  virtual std::string GetSuperPartitionName(uint32_t slot) = 0;
 };
 
 }  // namespace chromeos_update_engine