Exit app after a wallpaper is applied

Video for all apply button: https://drive.google.com/file/d/14hlFH42wULH2IEl96xD6ReOf-gvb05Bw/view?usp=sharing
The daily rotatio button also exit app.

Test: Manually
Bug: 157951178
Change-Id: I171275555925038b28ca31f058b8a5958fb2534f
diff --git a/src/com/android/wallpaper/module/DefaultWallpaperPersister.java b/src/com/android/wallpaper/module/DefaultWallpaperPersister.java
index 2e23338..b5cab4d 100755
--- a/src/com/android/wallpaper/module/DefaultWallpaperPersister.java
+++ b/src/com/android/wallpaper/module/DefaultWallpaperPersister.java
@@ -491,8 +491,8 @@
     @Override
     public void onLiveWallpaperSet() {
         android.app.WallpaperInfo currentWallpaperComponent = mWallpaperManager.getWallpaperInfo();
-        android.app.WallpaperInfo previewedWallpaperComponent =
-                mWallpaperInfoInPreview.getWallpaperComponent();
+        android.app.WallpaperInfo previewedWallpaperComponent = mWallpaperInfoInPreview != null
+                ? mWallpaperInfoInPreview.getWallpaperComponent() : null;
 
         // If there is no live wallpaper set on the WallpaperManager or it doesn't match the
         // WallpaperInfo which was last previewed, then do nothing and nullify last previewed