Introduce Window.DecorCallback

This is a follow up CL to my previous CL [1], which introduced a
hidden callback method into Window.Callback.

This CL does not change any observable behavior but consolidate that
hidden callback into a newly introduced Window.DecorCallback
interface.  In this approach, we no longer need to worry about
accidental method name collision in Window.Callback implementations
classes.

Here is the new callback chain.

 ViewRootImpl#performTraversals()
  -> DecorView#onSystemBarAppearanceChanged()
   -> Window.dispatchOnSystemBarAppearanceChanged()
    -> NavigationBarController.Impl#onSystemBarAppearanceChanged()

 [1]: I08566034bebfafff6777ce0152cd6ca1f66f6cad
      678ddce1

Bug: 215549533
Test: Manually tested with ThemedNavBarKeyboard sample
  1. Build aosp_coral-userdebug and flash it
  2. adb root
  3. adb shell setprop \
      persist.sys.ime.can_render_gestural_nav_buttons true
  4. adb reboot
  5. make -j ThemedNavBarKeyboard
  6. adb install -r \
      $OUT/system/app/ThemedNavBarKeyboard/ThemedNavBarKeyboard.apk
  7. adb shell ime enable \
      com.example.android.themednavbarkeyboard/.ThemedNavBarKeyboard
  8. adb shell ime set \
      com.example.android.themednavbarkeyboard/.ThemedNavBarKeyboard
  9. Open the Dialer app
 10. Focus in the top edit field.
 11. Tap "EXTENDED LIGHT NAVIGARION BAR" mode
 12. Make sure that the navigation button color is optimized for light
     navigation bar.
 13. Tap "STANDARD LIGHT NAVIGARION BAR" mode
 14. Make sure that the navigation button color is optimized for light
     navigation bar.
Change-Id: I5168f76dfc9526e925c565dbfa87597b5543fcdc