Also clear required_size if successful

On a VAB device, when sideloading a full package, if the device
ended up using the semi-flashing path (i.e. source partitions
are overwritten), clear required_size in the successful case as well.

Test: update_engine_unittests
Bug: 153555889
Change-Id: I881f578806b4bd5486f9d0d3b3cd631b7a752cc9
diff --git a/dynamic_partition_control_android.cc b/dynamic_partition_control_android.cc
index bf31244..20e02ac 100644
--- a/dynamic_partition_control_android.cc
+++ b/dynamic_partition_control_android.cc
@@ -493,8 +493,13 @@
     }
   }
 
-  return PrepareDynamicPartitionsForUpdate(
-      source_slot, target_slot, manifest, delete_source);
+  TEST_AND_RETURN_FALSE(PrepareDynamicPartitionsForUpdate(
+      source_slot, target_slot, manifest, delete_source));
+
+  if (required_size != nullptr) {
+    *required_size = 0;
+  }
+  return true;
 }
 
 namespace {