Clear quick settings backgrounds, update clock text color

- remove qs gradient background
- remove qs status bar black background
- update qs status bar clock onColorsChanged (match notif label colors)
- update status bar clock onDarkChanged

Bug: 172289783
Test: visual
   - qs & qs header background clear
   - observe sb clock - wallpaper light ? text dark : text white
   - observe qs clock - scrim light ? text dark : text white

Change-Id: Iff0aad81511c2cbc9ed9ecfcda03fda45d5001ae