Keep wallpaper in prepare back transition.

Revert part of ag/29115929, it would be safe as long as the wallpaper
surface won't merged into next transition.

Bug: 365500016
Flag: com.android.window.flags.migrate_predictive_back_transition
Test: atest BackAnimationControllerTest
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:a3d5df4aaac6d687cde0a31f0e22324a6ac6f9db)
Merged-In: I51232a95a585c1b931a68f456d9f208a6c50cb71
Change-Id: I51232a95a585c1b931a68f456d9f208a6c50cb71
diff --git a/libs/WindowManager/Shell/src/com/android/wm/shell/back/BackAnimationController.java b/libs/WindowManager/Shell/src/com/android/wm/shell/back/BackAnimationController.java
index 3e5adf3..5836085 100644
--- a/libs/WindowManager/Shell/src/com/android/wm/shell/back/BackAnimationController.java
+++ b/libs/WindowManager/Shell/src/com/android/wm/shell/back/BackAnimationController.java
@@ -1501,10 +1501,6 @@
                 int rootIdx = -1;
                 for (int i = info.getChanges().size() - 1; i >= 0; --i) {
                     final TransitionInfo.Change c = info.getChanges().get(i);
-                    if (c.hasFlags(FLAG_IS_WALLPAPER)) {
-                        st.setAlpha(c.getLeash(), 1.0f);
-                        continue;
-                    }
                     if (TransitionUtil.isOpeningMode(c.getMode())) {
                         final Point offset = c.getEndRelOffset();
                         st.setPosition(c.getLeash(), offset.x, offset.y);