Support light navigation bar

In order to propagate the information about whether the IME is using
light navigation bar or not from

  android.view.ViewRootImpl

to

  android.inputmethodservice.NavigationBarController

within the same process, this CL adds a new @hide method to

  Window.Callback

as follows.

 ViewRootImpl#performTraversals()
  -> DecorView#onSystemBarAppearanceChanged()
   -> Window.Callback#onSystemBarAppearanceChanged()
    -> SoftInputWindow#onSystemBarAppearanceChanged()
     -> NavigationBarController#onSystemBarAppearanceChanged()

Button color transition will be implemented in a subsequent CL.

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: I08566034bebfafff6777ce0152cd6ca1f66f6cad