Support nav button color transitions

With this CL, the back and IME switcher buttons on the navigation bar
have the same color transition even when they are rendered by the IME.

Note that the behavior is still behind a flag

  persist.sys.ime.can_render_gestural_nav_buttons,

which is still off by default.  There should be no observable changes
no matter whether the flag is enabled or disabled though.

Fix: 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 changes from light
     to dark with animation.
 13. Tap "EXTENDED DARK NAVIGARION BAR" mode
 12. Make sure that the navigation button color changes from dark
     to light with animation.
Change-Id: I41a3478a97d67b238c4424d93acbcad646aee0d2