fs_mgr: Make kDefaultAndroidDtDir constexpr

Change its type from std::string to constexpr char[] so that it does
not need to be constructed at runtime, which pollutes the bss page
unnecessarily.

Bug: 138856262
Test: Along with another fix for libbootloader_message.so, see that bss
      section for libbootloader_message.so is now clean on cuttlefish
      for several processes.

Change-Id: I1c8ff9cfeb25164a352cef89cfa7be889b3d3387