GitLab

* Introduced DesktopController interface. app_shell's Init process stays in ShellDesktopController
* Removed ShellAppWindowController
* Athena has its own AthenaDesktopController
* Intorduced AthenaEnv. This is now used for both AthenaMain and Unit test.
* Removed ScreenManagerDelegate and integrate it into AthenaEnv.
* Moved FocusController to ScreenManager, and removed CreateFocusRules()

BUG=397167
TBR=sky@chromium.org

Review URL: https://codereview.chromium.org/480353006

Cr-Commit-Position: refs/heads/master@{#291763}