Structure for screenshot detection notification.

The notification itself won't be hooked up until android U.

- The message container now has two (View.GONE) children, one for work
  profile first run, the other for screenshot detection.
- All interactions with this space go through
  MessageContainerController.
- Populating the message space has two phases: figuring out which
  messages want to be shown, then showing no more than one. These are
  implemented as separate methods on each of
  ScreenshotDetectionController and WorkProfileMessageController.
  MessageContainerController no longer knows the details of these views.
- ScreenshotDetectionController doesn't do the actual detection yet, but
  the UI bits have been manually tested.
- Moved the MessageContainerController.onScreenshotTaken calls to right
  after the entrance animation completes in ScreenshotController.

Flags: Added a new SCREENSHOT_DETECTION flag. The detection and relevant
       display code are protected by this. All other changes to the
       message space are protected by the work profile flag.
Test: atest com.android.systemui.screenshot
Bug: 265937777
Change-Id: Id1e51ffcb322839badf4f1d81a5aafceb0b67a06