Enable IMS#canImeRenderGesturalNavButtons() by default

With this CL, the back button and IME switcher button on the
navigation bar are rendered by the IME when gestural navigation mode
is enabled.

The new behavior can still be manually disabled as follows

  adb root
  adb shell setprop  \
        persist.sys.ime.can_render_gestural_nav_buttons false
  adb reboot

The only observable behavior change we are aware of is that now those
buttons are included in the task screenshot in Recents.

Other than that, there should be no observable behavior change.

Fix: 215545985
Test: Manually tested with ThemedNavBarKeyboard sample
  1. Build aosp_coral-userdebug and flash it
  2. Enable gesture navigation mode.
  3. make -j ThemedNavBarKeyboard
  4. adb install -r \
      $OUT/system/app/ThemedNavBarKeyboard/ThemedNavBarKeyboard.apk
  5. adb shell ime enable \
      com.example.android.themednavbarkeyboard/.ThemedNavBarKeyboard
  6. adb shell ime set \
      com.example.android.themednavbarkeyboard/.ThemedNavBarKeyboard
  7. Open the Dialer app
  8. Tap "EXTENDED LIGHT NAVIGARION BAR" mode
  9. Make sure that the navigation button color is optimized for light
     navigation bar.
 10. Tap "STANDARD LIGHT NAVIGARION BAR" mode
 11. Make sure that the navigation button color is optimized for light
     navigation bar.
 12. Tap "FLOATING MODE" mode
 13. Make sure that the navigation buttons are visible.
Change-Id: I45e511f5cfec93cdd002d23c091b4fe735b28227