Moar C++ refactoring.

- Use std::string on runCommand(), DurationReport, dumpFile() and their
  dependencies.
- Moves runCommand() and dumpFile() to Dumpstate class.

These 2 commands are the only ones currently used on dumpstate_board() implementations.

BUG: 26379932
Test: manual / refactored code

Change-Id: I09072d4e183851f90c07960d1f971a925ea2c801