Update the IME nav UI when gesture navigation mode changes

With this CL, nav buttons rendered by InputMethodService can be
dynamically enabled or disabled when the gesture navigation mode is
updated.

Note that the feature is still disabled by default behind the flag.
Hence there should be no observable behavior changes unless the
feature is explicitly enabled with setprop.

Fix: 215552600
Test: Manually done as follows
  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. Tap any edit field to show AOSP Keyboard.
  6. adb shell dumpsys input_method | grep mNavigationBarController
     -> Confirm mRenderGesturalNavButtons is false.
     -> Confirm mNavigationBarFrame is null.
  7. Switch to gestural navigation mode.
  8. adb shell dumpsys input_method | grep mNavigationBarController
     -> Confirm mRenderGesturalNavButtons is true.
     -> Confirm mNavigationBarFrame is not null.
  9. Switch to 3-button navigation mode again.
 10. adb shell dumpsys input_method | grep mNavigationBarController
     -> Confirm mRenderGesturalNavButtons is false.
     -> Confirm mNavigationBarFrame is null.

Change-Id: Id0cfa44cce5de515dc5d28254e1d41bdfc01e201