otautil: Clean up dirCreateHierarchy().

- Changed to std::string based implementation (mostly moved from the
  former make_parents() in updater/install.cpp);
- Removed the timestamp parameter, which is only neeed by file-based OTA;
- Changed the type of mode from int to mode_t;
- Renamed dirCreateHierarchy() to mkdir_recursively().

Test: recovery_unit_test passes.
Test: No external user of dirCreateHierarchy() in code search.
Change-Id: I71f8c4b29bab625513bbc3af6d0d1ecdc3a2719a