Improved the gradient drawing of the media notification

Previously we were drawing with a color on top of the
image instead of fading it out. This could lead to
visible artefacts in the background.
We're now fading out the alpha in a similar manner instead.

Test: runtest systemui
Change-Id: Ie6af7751a734b8fa44279eba970e7ca5ba67a4c0
Fixes: 37950482