D-Bus API: Add GetDataPlans, DataPlansUpdate, and RequestDataPlansUpdate

Implement "the real" D-Bus API for use by libcros. Returns hardcoded test data for now.

The related CL for libcros changes is: http://codereview.chromium.org/3578003/show

BUG=chromium-os:3510
TEST=libcros/monitor_network

Change-Id: I50530f90a083f6bdba3ef15c718c43e94e308f50

Review URL: http://codereview.chromium.org/3595001