FAFT: Move wait_for_client and wait_for_client_offline to ModeSwitcher

These two methods are tightly related to ModeSwitcher. This change
helps it to be derived to different implementations.

BUG=chrome-os-partner:39744
TEST=Ran some of the effected FAFT tests.

Change-Id: Id9be1decec3ebc5a843a30b3070a2352546423eb
Reviewed-on: https://chromium-review.googlesource.com/294773
Commit-Ready: Wai-Hong Tam <waihong@chromium.org>
Tested-by: Wai-Hong Tam <waihong@chromium.org>
Reviewed-by: danny chan <dchan@chromium.org>
17 files changed