Don't load wallpaper colors if no listener set

Currently, we always load wallpaper color, then check if listener exist.

Test: Manually
Bug: 159195012
Change-Id: Ice8505927fead23a53b15448f192689c02cb3e5c
diff --git a/src/com/android/customization/picker/WallpaperPreviewer.java b/src/com/android/customization/picker/WallpaperPreviewer.java
index 4993a56..9e56cec 100644
--- a/src/com/android/customization/picker/WallpaperPreviewer.java
+++ b/src/com/android/customization/picker/WallpaperPreviewer.java
@@ -176,14 +176,12 @@
                 }
 
                 // Load wallpaper color for static wallpaper.
-                WallpaperColorsLoader.getWallpaperColors(
-                        mActivity,
-                        mWallpaper.getThumbAsset(mActivity),
-                        colors -> {
-                            if (mWallpaperColorsListener != null) {
-                                mWallpaperColorsListener.onWallpaperColorsChanged(colors);
-                            }
-                        });
+                if (mWallpaperColorsListener != null) {
+                    WallpaperColorsLoader.getWallpaperColors(
+                            mActivity,
+                            mWallpaper.getThumbAsset(mActivity),
+                            mWallpaperColorsListener::onWallpaperColorsChanged);
+                }
             }
         }
     }