Extract colors from wallpaper and dynamically update login screen overlay

ui/login/account_picker/md_screen_account_picker.css

Index: ui/login/account_picker/md_screen_account_picker.css
diff --git a/ui/login/account_picker/md_screen_account_picker.css b/ui/login/account_picker/md_screen_account_picker.css
index ca8742c8c0b174b376f22ea5b939cdd93cccd115..e2a2d54dfb080f53a3bce6afb33deddc39804d05 100644
--- a/ui/login/account_picker/md_screen_account_picker.css
+++ b/ui/login/account_picker/md_screen_account_picker.css
@@ -74,6 +74,10 @@ html[screen=lock] #signin-banner.message-set {
   position: absolute;
 }
 
+.small-pod-container.images-loading {
+  visibility: hidden;
+}
+
 .small-pod-container::-webkit-scrollbar {
    width: 0;
 }
@@ -99,6 +103,10 @@ html[screen=lock] #signin-banner.message-set {
   z-index: 2;
 }
 
+.small-pod-container-mask.images-loading {
+  visibility: hidden;
+}
+
 .small-pod-container-mask.rotate {
   transform: rotate(180deg);
 }