Introduce system_info as an independent container.

- system info persists on the screen, not in the background
- introduce a new container to overwrap with home card nicely
- conflicting with overview windows doesn't look correct -- therefore
  its y-coordinate should be moved by the height of system info.
- system_ui's "ColorScheme" isn't used anymore, since it's always
  white (LIGHT) colors in the latest mock. I'll create another CL
  to remove this.

BUG=425724
R=oshima@chromium.org
TEST=manually

Review URL: https://codereview.chromium.org/715053003

Cr-Commit-Position: refs/heads/master@{#304043}

• athena/athena.gyp[diff]
• athena/home/DEPS[diff]
• athena/home/home_card_constants.cc[diff]
• athena/home/home_card_constants.h[diff]
• athena/home/home_card_unittest.cc[diff]
• athena/home/home_card_view.cc[diff]
• athena/system/DEPS[diff]
• athena/system/background_controller.cc[diff]
• athena/system/public/system_ui.h[diff]
• athena/system/system_ui_impl.cc[diff]
• athena/util/athena_constants.cc[Added - diff]
• athena/util/athena_constants.h[Added - diff]
• athena/util/container_priorities.h[diff]
• athena/wm/window_overview_mode.cc[diff]