system_api: Add powerd ScreenIdleState proto.

Add a ScreenIdleState protocol buffer message that will be
included in new ScreenIdleStateChanged D-Bus signals emitted
by powerd.

Also add a "reason" enum field to SuspendImminent.

BUG=chromium:778449
TEST=built it

Change-Id: I692015498c258e47bba3d5c4e429596cb17fa16e
Reviewed-on: https://chromium-review.googlesource.com/740664
Commit-Ready: Dan Erat <derat@chromium.org>
Tested-by: Dan Erat <derat@chromium.org>
Reviewed-by: Eric Caruso <ejcaruso@chromium.org>
4 files changed