libbrillo: Add functionality to get the app id of a kiosk app.

This CL adds the functionality to libpolicy to return the ID of the
auto-launched kiosk app, if one exists in the device policy.

BUG=chromium:707892
TEST=automated

Change-Id: Ia8fd1836c288be8a12a7b3fbd2e064e71cd118fe
Reviewed-on: https://chromium-review.googlesource.com/486241
Commit-Ready: Rahul Chaturvedi <rkc@chromium.org>
Tested-by: Rahul Chaturvedi <rkc@chromium.org>
Reviewed-by: Rahul Chaturvedi <rkc@chromium.org>
(cherry picked from commit d6d6f31b3988e21d0a5b50977ef7befedf339b55)
9 files changed