autotest: Refactor iw related code into a delegate

This allows us to easily reuse our interfaces to iw, such as
the parsing of iw list.  The iw list parsing has been improved
to additionally parse supported 802.11n MCS indices.

TEST=wifi_matfunc continues to pass with these changes
BUG=chromium:308326

Change-Id: I92da1174d981bf0ba5f97227b68157ce9c2a6ecb
Reviewed-on: https://chromium-review.googlesource.com/173642
Reviewed-by: Christopher Wiley <wiley@chromium.org>
Tested-by: Christopher Wiley <wiley@chromium.org>
Commit-Queue: David James <davidjames@chromium.org>
4 files changed