Fix NPE when leaving kids mode

Do not apply leash operations if we are leaving kids mode since the root task will be deleted.

Bug: 242873681
Test: 1. Select 3-button nav mode
      2. enable kids mode by below command:
         adb shell setting put secure nav_bar_kids_mode 1
      3. Swtich to gesture nav mode to exit kids mode and no crash
Change-Id: I04cab6854a40bcae42b8c83db2e6866e4f558cca