md-backdrop: Animate opacity instead of background-color

[websirnik]

I've noticed that animating opacity was in plans #3841 (comment), but then the issue was closed without the necessary commit.
In 10.1 since backdrop became darker, you can easily see that animation does not perform well. Animating opacity instead of background-color will improve the performance.

[ThomasBurleson]

Fixed in SHA a2cb6cf