flashing: unlock block partitions before installing a zip

Change-Id: I48b3db34116a9097a9206162468bc009993ef7d1
(cherry picked from commit d56818f5af1807d388657ba60b0900c79bcbc05b)
diff --git a/twrpinstall/twinstall.cpp b/twrpinstall/twinstall.cpp
index 7975a97..6805a2f 100755
--- a/twrpinstall/twinstall.cpp
+++ b/twrpinstall/twinstall.cpp
@@ -342,6 +342,7 @@
 			if (!system_mount_state)
 				PartitionManager.UnMount_By_Path(PartitionManager.Get_Android_Root_Path(), false);
 			if (android::base::GetBoolProperty("ro.virtual_ab.enabled", false)) {
+				PartitionManager.Unlock_Block_Partitions();
 				PartitionManager.Prepare_All_Super_Volumes();
 				gui_warn("mount_vab_partitions=Devices on super may not mount until rebooting recovery.");
 			}