Modernize mtpd source.

- Use C++11 features like nullptr and type aliasing via using.
- Remove an obsolete TODO.
- Constify some DeviceManager member pointers.

BUG=none
TEST=none

Change-Id: Iac0cdf626420f815f11c6b6792ca60581cd14668
Reviewed-on: https://chromium-review.googlesource.com/489482
Commit-Ready: Lei Zhang <thestig@chromium.org>
Tested-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
4 files changed