[login_manager] Add stub DBus API for policy get/set

Just adding the DBus calls for getting/setting a policy to the session manager.

BUG=12670
TEST=build the code; methods do nothing as of yet.

Change-Id: I53a5992c813adfda20865ceff583c78a4e56fc51

Review URL: http://codereview.chromium.org/6591109
6 files changed