Support floating IMEs even when IMEs render the nav buttons

As discussed in Bug 201375975, the nevigation bar handling for
floating IMEs is a bit tricky.

This CL tweaks DecorView behabior only when

  InputMethodService#canImeRenderGesturalNavButtons(),

is true and the IME is floating in the gestural navigation mode.

Fix: 215550296
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 "FLOATING MODE" mode
 12. Make sure that the navigation buttons are visible.
Change-Id: Ia54499a3c2ac6e33e72f625eba3477fd81649d32