Merge "Use more std::string in fastboot." am: 17f3b1bad7 am: 05ec41963d
am: a73dfe0cf9

* commit 'a73dfe0cf991d45803cf14a0fcd32154d6dca974':
  Use more std::string in fastboot.

Change-Id: Ia585a2ebf1aa88bd3e48ce68d35216f422bdf925