Exclude nav bar area from apps bounds when in kids mode

GKS requested that app's bounds should not overlap with the nav bar area
when kids mode is enabled except for Launcher.

Creates a dedicate task organizer for kids mode. When kids mode is
enabled, create a root task with its bounds excluding the nav bar area
and tasks except launcher will be launched into it.

Bug: 161689887
Test: manual:
      1. adb shell settings put secure "nav_bar_force_visible" "1"
      2. choose 3-buttons navigation mode in
         Settings>System>Gestures>System navigation
      3. check winscope and see if bounds are set correctly
Test: atest KidsModeTaskOrganizerTest
Test: atest WindowOrganizerTests

Change-Id: I93af1bc5b2696654d8ca5055dbbbebf5a41f89aa