Rewrite TWRP fstab flag processing

* Do not flip/flop between std::string and char* when no std::string
  specific functions are applied.
* Remove the need to manually count flag string lengths
* Move checks for Display_Name, Storage_Name, and Backup_Display_Name
  out of flag processing

Change-Id: I24d432c222124012b2a98d27598e42d0944f7da2