DND: Add a hidden DND tile to quick settings.

Invisible by default, not activatable when the platform
volume controller is active.

However, when the platform volume controller is not active,
it can be enabled via a broadcast intent.

When enabled, the status bar icon also changes to a single
dnd icon.

Very little more than embedding the existing zen mode panel
into the detail pane of a new QS tile.

Change-Id: I1e0ff6fbb99b00f67b53bceda8cbf121f3ef6b52