DynamicPartitionControl: Add required_size to Prepare

Add out parameter required_size to PreparePartitionsForUpdate
to indicate the total size required on /userdata in order
to apply the update.

Bug: 138808058
Test: update_engine_unittests

Change-Id: I2768d13671e212fd24a1a22811b50c9738834459
diff --git a/common/dynamic_partition_control_stub.h b/common/dynamic_partition_control_stub.h
index 1704f05..09990a7 100644
--- a/common/dynamic_partition_control_stub.h
+++ b/common/dynamic_partition_control_stub.h
@@ -35,7 +35,8 @@
   bool PreparePartitionsForUpdate(uint32_t source_slot,
                                   uint32_t target_slot,
                                   const DeltaArchiveManifest& manifest,
-                                  bool update) override;
+                                  bool update,
+                                  uint64_t* required_size) override;
 
   bool FinishUpdate() override;
 };