Fixed fading and dozemode for custom notifications

The custom notifications were fading really ugly when they
had a dark background like media notifications, because
it was fading from dark to dark.
Now this background is shared for both custom views which also
reduces overdraw for them.

In addition does the doze mode now work much nicer because we're
only fading them to greyscale instead of inverting.
This also fixed an issue where legacy custom notifications with
a dark background were colorful during doze.

Bug: 19437552
Change-Id: I87798da9ac11b9abfe4470b6ca53b555da3aa629