Fix 'No OS' on System_Image restore after wipe

* When you format the system partition, it will remain
  in a mounted state.
* Subsequently restoring a system_image (even though
  successfully) the Update_System_Details() function
  will not correctly update the system partition
  details.

* Reproducible by:
  1- Advanced wipe: System
  2- Restore: System_Image
  3- Reboot -> No OS prompt

* eg: [
       ~ # twrp get tw_min_system
           tw_min_system = 50
       ~ # twrp get tw_backup_system_size
           tw_backup_system_size = 8
       ~ # mount -o ro /system
       ~ # du -sh /system
           3.5G    /system
      ]

Change-Id: I99f75274816788dd38eccdd387f7ac691e1f3fab

partitionmanager.cpp

@@ -999,6 +999,7 @@ int TWPartitionManager::Run_Restore(const string& Restore_Name) {
 		}
 	}
 	TWFunc::GUI_Operation_Text(TW_UPDATE_SYSTEM_DETAILS_TEXT, gui_parse_text("{@updating_system_details}"));
+	UnMount_By_Path("/system", false);
 	Update_System_Details();
 	UnMount_Main_Partitions();
 	time(&rStop);