Animate QS alpha on split shade pull down

When pulling down the shade in split mode, the quick settings alpha
should be directly connected to the drag down gesture and the same that
we have for the notification shade.

Test: manual
Bug: 200804740
Fixes: 196722669
Change-Id: I6532c8a78826a5b60609dc66557abc49a3f140c5
Merged-In: I6532c8a78826a5b60609dc66557abc49a3f140c5