Transitioning media on lockscreen with a fade

UX wise a fade was much preferred, so we built in the capability
for media transitions to fade from location to location.
We're now also fading the media when transitioning
between QS and Lockscreen.

This also polishes the animation further.

Bug: 184946919
Test: atest SystemUITests
Change-Id: Id9fc58469bebe69ad7a0189e4c4acd36523cdeed