Add trace to generateCrop in wallpaperManagerService

generateCrop does some IO operations and could be responsible for some lock contention; it is useful to know how long it takes to run.

Test: treehugger
Bug: 247076432
Change-Id: I710641d3b15eeca7a34cf6a6b508e769b6604532

services/core/java/com/android/server/wallpaper/WallpaperManagerService.java

@@ -599,6 +599,13 @@ public class WallpaperManagerService extends IWallpaperManager.Stub
      * for display.
      */
     void generateCrop(WallpaperData wallpaper) {
+        TimingsTraceAndSlog t = new TimingsTraceAndSlog(TAG);
+        t.traceBegin("WPMS.generateCrop");
+        generateCropInternal(wallpaper);
+        t.traceEnd();
+    }
+
+    private void generateCropInternal(WallpaperData wallpaper) {
         boolean success = false;
 
         // Only generate crop for default display.