Avoid potential NPE if the restore-below task is not collected

If there is an existing collecting transition, the restore-below
task may not be collected.

Also make sure continueWindowLayout() will be called in finally
block. Previously if handleStartResult() throws exception, the
surface layout will be broken that its defer count isn't paired.

Bug: 230582311
Test: adb shell setprop persist.wm.debug.shell_transit 1; reboot
      Launch/swipe app quickly many times.
Change-Id: I7ae962c5085630a5cf194625d2f8b3e8f30a45b2