Revert "Remove background drawing behind notifications"

This reverts commit 7552019d.

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