Add camera affordance to navigation bar on phones

This adds a camera button on phones that can be used to show
and launch the camera.

- Minor refactoring of touch event dispatch in PagedView.
- Disables usability hints when keyguard loads.
- Only add a touch handler for camera icon once during layout.
- Update after review.
- Updated with latest UX camera and camera background assets

Change-Id: I09cd5cb0e501fd0f4659bea96d00c92b07f805c4