PartitionSizes -> PartitionMetadata

In a following CL, InitPartiitonMetadata not only
takes sizes but also group of each partition. Rename
the struct to a more generic name.

Test: builds
Change-Id: Ie8a7e58b0cccc45e533de368e24f5a73d556de65
diff --git a/payload_consumer/delta_performer.cc b/payload_consumer/delta_performer.cc
index f7323f9..ca898d5 100644
--- a/payload_consumer/delta_performer.cc
+++ b/payload_consumer/delta_performer.cc
@@ -919,12 +919,12 @@
   }
 
   if (install_plan_->target_slot != BootControlInterface::kInvalidSlot) {
-    BootControlInterface::PartitionSizes partition_sizes;
+    BootControlInterface::PartitionMetadata partition_metadata;
     for (const InstallPlan::Partition& partition : install_plan_->partitions) {
-      partition_sizes.emplace(partition.name, partition.target_size);
+      partition_metadata.emplace(partition.name, partition.target_size);
     }
     if (!boot_control_->InitPartitionMetadata(install_plan_->target_slot,
-                                              partition_sizes)) {
+                                              partition_metadata)) {
       LOG(ERROR) << "Unable to initialize partition metadata for slot "
                  << BootControlInterface::SlotName(install_plan_->target_slot);
       *error = ErrorCode::kInstallDeviceOpenError;