Add intent extra when launching WallpaperPicker from Launcher

 - Add extra to the intent to help WallpaperPicker log the launch
 source
 - design doc: go/log-wppg-launch-source

Bug: 154781896
Test: manually
Change-Id: Ife774866b06251c01f28f72555c3dcc844fdc051
diff --git a/src/com/android/launcher3/Utilities.java b/src/com/android/launcher3/Utilities.java
index bf63788..681118d 100644
--- a/src/com/android/launcher3/Utilities.java
+++ b/src/com/android/launcher3/Utilities.java
@@ -141,6 +141,10 @@
     public static final String EXTRA_WALLPAPER_OFFSET = "com.android.launcher3.WALLPAPER_OFFSET";
     public static final String EXTRA_WALLPAPER_FLAVOR = "com.android.launcher3.WALLPAPER_FLAVOR";
 
+    // An intent extra to indicate the launch source by launcher.
+    public static final String EXTRA_WALLPAPER_LAUNCH_SOURCE =
+            "com.android.wallpaper.LAUNCH_SOURCE";
+
     public static boolean IS_RUNNING_IN_TEST_HARNESS =
                     ActivityManager.isRunningInTestHarness();