Uses a listener on wallpaper manager to get the wallpaper colors for
fallback home. When the colors are ready it can correctly set the system
ui flags.
Change-Id: I7119c0f035245539cb69bec1ccccecef64d3aff5
Fixes: 79776583
Test: reboot phone, unlock with pin/pattern with light them wallpaper
Test: make RunSettingsRoboTests ROBOTEST_FILTER=FallbackHomeActivityTest
