Don't fade away clock when QS expands

QS isn't transparent anymore, the clock shouldn't fade away.
Fading out the clock was introduced in O-MR1 to avoid the contrast
between the white clock and the wallpaper.

Test: visual
Test: drag panel until bouncer almost shows up, release it
Bug: 73830624
Change-Id: I8b31a40cebf7dc2109db34cafc2701925fceabf5