Quantum notification improvements.

New API introduced here: Notification.color (and
Builder.setColor()), allowing apps to specify an accent
color to be used by the template. The Quantum templates
(which are now the only kind we support) use this when
creating a circular background to draw behind the smallIcon
in the expanded form.

Additionally, the quantum and legacy templates are no longer
in superposition; all apps using Builder will get quantum.

Change-Id: Iac5e2645cc5c2346ed458763f2280ae9c6368b62