bootloader_message: Remove global std::string

A global std::string, even if not used, pollutes the bss section
unnecessarily.  Since this object is only there for testing, make it
std::optional<std::string>, which is constexpr constructible.

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

Change-Id: I6df837dded88d979ffe14d5b2770b120bcf87341