Support the ability to force-show navigation for GKS

GKS requested to force show nav bar except for Launcher apps.

Returns null in InsetsPolicy.getNavControlTarget() to forcibly
show the nav bar when:
  - The feature is enabled.
  - The activity type of the focus window is ACTIVITY_TYPE_STANDARD.

Bug: 161689887
Test: manual:
      1. adb shell settings put secure "nav_bar_kids_mode" "1"
      2. choose 3-buttons navigation mode in
         Settings>System>Gestures>System navigation
      2. open an app that hides nav bar and check if the nav bar is
         still visible.
Change-Id: I7679da7d8b0892317d04cdc3553fe9a8fcdb2292