Change SuperImageProperties.Block_devices to be a slice of string

There are some products have empty BOARD_SUPER_PARTITION_BLOCK_DEVICES,
which is valid. But the previous super image logic didn't handle
it correctly. This CL fixes the issue.

Test: CI and verified on the product that has PRODUCT_BUILD_SUPER_PARTITION
      true and empty BOARD_SUPER_PARTITION_BLOCK_DEVICES
Change-Id: I761848ce6abc3f75a5b2083b79008f236e08b4fb
diff --git a/fsgen/super_img.go b/fsgen/super_img.go
index 4569896..8ee3bf2 100644
--- a/fsgen/super_img.go
+++ b/fsgen/super_img.go
@@ -35,7 +35,7 @@
 
 	superImageProps := &filesystem.SuperImageProperties{
 		Metadata_device:        proptools.StringPtr(partitionVars.BoardSuperPartitionMetadataDevice),
-		Block_devices:          proptools.StringPtr(partitionVars.BoardSuperPartitionBlockDevices[0]),
+		Block_devices:          partitionVars.BoardSuperPartitionBlockDevices,
 		Ab_update:              proptools.BoolPtr(partitionVars.AbOtaUpdater),
 		Retrofit:               proptools.BoolPtr(partitionVars.ProductRetrofitDynamicPartitions),
 		Virtual_ab:             proptools.BoolPtr(partitionVars.ProductVirtualAbOta),