Show icons of only background apps in Toasts

As implemented and discussed in I82bf23664eea134f3b1f89ad5a99f6be73906ba8,
this patch disables the icons in Tosts for already foreground apps. It's
useful, no questions about that but please be aware of:

- Icons in Toasts are also displayed in "tooltips" of ActionBar (when you
press and hold an action). I think that goes agains material design as the
ActionBar component now doesn't even show app icon.
- Toasts from Android OS contain the Green robot. (IMHO: It was never intended
for system Toast to have an icon)
- Toasts are intended "to be as unobtrusive as possible" and this kinds of
break it. (From javadoc)
- Icons should stay as workaround for finding that badly designed/intrusive
app because: "Your app should not create a dialog or toast if it is not
currently on screen." (From notifications design paterns)
- Not that relevant but not all users welcome this aesthetic:
https://www.reddit.com/r/cyanogenmod/comments/3oazh8/does_the_icon_in_the_toast_notification_bother/
https://www.reddit.com/r/cyanogenmod/comments/3jg5c2/icons_on_toasts/
etc.

Change-Id: If748297bd422c7ddbf061da735b8e4ed0a41b425