Chameleon: Add factory class to generate display_client

The new factory class MultimediaClientFactory generates the original
DisplayClient object to access DUT diplay-related methods.

The XML RPC connection is abstracted into the MultimediaClientConnection
class.

BUG=chromium:407004
TEST=Ran the Chameleon test display_Resolution.mirrored it passed.

Change-Id: I14d16450a54be08c83ccb691a0fbd17186561374
Reviewed-on: https://chromium-review.googlesource.com/214424
Reviewed-by: Wai-Hong Tam <waihong@chromium.org>
Tested-by: Wai-Hong Tam <waihong@chromium.org>
Commit-Queue: Cheng-Yi Chiang <cychiang@chromium.org>
3 files changed