Add DynamicPartitionControlInterface::CleanupSuccessfulUpdate

This is a wrapper over SnapshotManager::WaitForMerge. It waits until the
previous update is merged, then return.

Bug: 138808328
Test: manual with update_engine_client

Change-Id: If44854810f37dd959ffdf3f62f26528867a71fc8
diff --git a/mock_dynamic_partition_control.h b/mock_dynamic_partition_control.h
index ffabac7..1237d76 100644
--- a/mock_dynamic_partition_control.h
+++ b/mock_dynamic_partition_control.h
@@ -43,6 +43,7 @@
       bool(uint32_t, uint32_t, const DeltaArchiveManifest&, bool, uint64_t*));
   MOCK_METHOD0(GetVirtualAbFeatureFlag, FeatureFlag());
   MOCK_METHOD0(FinishUpdate, bool());
+  MOCK_METHOD0(CleanupSuccessfulUpdate, ErrorCode());
 };
 
 class MockDynamicPartitionControlAndroid