Refresh tablet notification drawer.

  * New "X" button appears in the system bar, allowing the
    user to clear all notifications. Only appears when the
    notifications list is showing and there are clearable
    items. (Note: the textual button on phones has also been
    replaced with an X.)

  * Updated panel background artwork and positioning.

  * Removed a bunch of old art.

Bug: 4970588
Bug: 4974043
Bug: 3509407 (regression)

Change-Id: Ibadb638cd18c4faa751cd1f311d73ceda65cb3ca