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/common/fake_boot_control.h b/common/fake_boot_control.h
index e71c83a..513c149 100644
--- a/common/fake_boot_control.h
+++ b/common/fake_boot_control.h
@@ -74,8 +74,8 @@
     return true;
   }
 
-  bool InitPartitionMetadata(Slot slot,
-                             const PartitionSizes& partition_sizes) override {
+  bool InitPartitionMetadata(
+      Slot slot, const PartitionMetadata& partition_metadata) override {
     return true;
   }