GitLab

* keep alive has to be incremented at startup.
* exclude more code that depends on ash.
* create a screen for shutdown

Minor cleanup
* remove all containers when ScreenManager is deleted.

BUG=397167, 401044
TEST=AthenaShutdownTest.Shutdown

Review URL: https://codereview.chromium.org/558823002

Cr-Commit-Position: refs/heads/master@{#294083}