SystemUI: Allow theming of the "no recents" label

This commit defines the label of the "no recent apps" label as a
simple color which allows it to be modified in themes. This resource
matches what CM has been using in cm_colors.xml for a while.

Change-Id: I1346eb25dbcab519f3794d6601337ffa3b632e35
Merge "SystemUI: Allow theming of the "no recents" label" into kitkat