SystemUI: disable wallpaper-based tint for scrim

This is quite annoying especially when the lockscreen is displaying an album art.
You don't really want to see the green of your amazing wallpaper
picturing a gorgerous forest landscape when you're rocking Master of Puppets.

Also several users and contributors complained about this thing.
Also it was removed in P dp1 and it never came back in later P builds.

[BadDaemon]
For dark text, use a fully transparent scrim so there is no negative impact
for that usecase
GlobalActionsDialog: Adapted and refactored code for less code change

Change-Id: I5ce5d5ff002f3be9b13a4cbb8df2300d0d7dcb3b
Signed-off-by: Joey <joey@lineageos.org>