An app will be able to show a window on lock screen only if it
was registered as a handler for "new_note" tray action, and a
user performed an action that causes the action to be handled.

Lock screen app windows can have two visible states:
 * active - when the window is displayed over the user pods on lock
       screen UI.
 * background - when the window is displayed in background, under the
       user pods in lock screen UI.

In both states, the lock screen UI system shelf should remain visible.
Lock screen app windows are currently restricted only to maximized
bounds.

To implement this behavior, this CL introduces a new container that
adjusts its children visibility state according to lock screen new
note action state:
 * children can be active only when the action is in active state
 * children can be visible only when the action is in active or
   background state
 * in background state, children are stacked below the lock screen
   window
 * in active state, children are stacked above lock screen window

BUG=716081

Review-Url: https://codereview.chromium.org/2876993002
Cr-Commit-Position: refs/heads/master@{#475042}