Ignore moveTaskToBack transition for PiP

YT calls Activity#moveTaskToBack(false) when tapping on background play
in PiP mode. Simply ignore any system transition in this case.

See also go/pip-move-task-to-back

Video: http://recall/-/aaaaaabFQoRHlzixHdtY/g1i3IAyUeSJua1dA61OhXQ
Bug: 175279237
Test: Tap background play when in PiP, see Video
Change-Id: I7319fe6e12208e51e516bc4ea82ee7325ccecc11

services/core/java/com/android/server/wm/Task.java

@@ -7176,8 +7176,11 @@ class Task extends WindowContainer<WindowContainer> {
         if (DEBUG_TRANSITION) Slog.v(TAG_TRANSITION, "Prepare to back transition: task="
                 + tr.mTaskId);
 
-        mDisplayContent.prepareAppTransition(TRANSIT_TO_BACK);
-        mDisplayContent.requestTransitionAndLegacyPrepare(TRANSIT_TO_BACK, tr);
+        // Skip the transition for pinned task.
+        if (!inPinnedWindowingMode()) {
+            mDisplayContent.prepareAppTransition(TRANSIT_TO_BACK);
+            mDisplayContent.requestTransitionAndLegacyPrepare(TRANSIT_TO_BACK, tr);
+        }
         moveToBack("moveTaskToBackLocked", tr);
 
         if (inPinnedWindowingMode()) {