Do not letterbox the activity in PiP mode

There are chances that a Letterbox is added to the Task entering PiP
mode, simply ignore such request.

Video: http://recall/-/aaaaaabFQoRHlzixHdtY/dPygRdWjyGltlCmQU0TPZn
Bug: 283008030
Test: Follow the reproduce steps in bug, see video
Change-Id: I83c5d32ef7cff4b3b6c85a698d068935c83c9423

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

@@ -3883,13 +3883,15 @@ class WindowState extends WindowContainer<WindowState> implements WindowManagerP
     }
 
     /**
-     * @return true if activity bounds are letterboxed or letterboxed for diplay cutout.
+     * @return {@code true} if activity bounds are letterboxed or letterboxed for display cutout.
+     * Note that it's always {@code false} if the activity is in pip mode.
      *
      * <p>Note that letterbox UI may not be shown even when this returns {@code true}. See {@link
      * LetterboxUiController#shouldShowLetterboxUi} for more context.
      */
     boolean areAppWindowBoundsLetterboxed() {
         return mActivityRecord != null
+                && !mActivityRecord.inPinnedWindowingMode()
                 && (mActivityRecord.areBoundsLetterboxed() || isLetterboxedForDisplayCutout());
     }