Revert "Remove background drawing behind notifications"

This reverts commit 7552019d8bf2033195b1046d9870a1b093574189.

Reason for revert: removing outline breaks lockscreen <=> aod notif animation

DIFF - R.color.notification_shade_background_color is not available
anymore, so get default notif panel background color this way:

Utils.getColorAttr(mContext, android.R.attr.colorBackground).getDefaultColor()

Fixes: 174665900
Change-Id: Ia939e6bc1cb80e8d44b5c00e1534ab36f2c641cc
diff --git a/packages/SystemUI/res/values/config.xml b/packages/SystemUI/res/values/config.xml
index 01b55b7..880dd378 100644
--- a/packages/SystemUI/res/values/config.xml
+++ b/packages/SystemUI/res/values/config.xml
@@ -357,6 +357,9 @@
          the notification is not swiped enough to dismiss it. -->
     <bool name="config_showNotificationGear">true</bool>
 
+    <!-- Whether or not a background should be drawn behind a notification. -->
+    <bool name="config_drawNotificationBackground">true</bool>
+
     <!-- Whether or the notifications can be shown and dismissed with a drag. -->
     <bool name="config_enableNotificationShadeDrag">true</bool>