Settings: Slimrecents background color && transparency [1/2]

Allow user to specify background color and transparency level
    for the slimrecents panel.

PS: now setting sticks through reboots and TRDS state
        changes. also, add reset to default color option.

PS: when resetting to default, use TRDS (png) resources
        instead of custom color

PS: properly set preference summary for default
        value ("TRDS default")

Change-Id: I8dd2543a3201b36f8c310de0da54f2f91ca474ef