Minimum impl of nav buttons rendered by IME

This is the minimum implementation of rendering the back button and
the IME switcher button in the InputMethodService process.

Note that the feature is still disabled by default behind the flag.
Newly added UI logic should be running only when the feature flag is
enabled.

 Bug 215545985: Enable canImeRenderGesturalNavButtons() by default

Here is the list of known blockers before enabling the flag.

 * Bug 215552600: Subscribe gesture mode change.
 * Bug 215551357: Show IME swicher button only when necessary.
 * Bug 215549533: Support light navigation bar.
 * Bug 215550296: Support floating IME.
 * Bug 215554582: Set "input_method_rounded_corner_content_padding"
                  for each devices.

Also, after enabling the flag by default, we start cleaning up newly
added code under

  android.inputmethodservice.navigationbar.

at Bug 215443343.

Bug: 205803355
Test: Manually done as follows
  1. Build aosp_coral-userdebug and flash it
  2. Switch to gestural navigation mode.
  3. adb root
  4. adb shell setprop \
       persist.sys.ime.can_render_gestural_nav_buttons true
  5. adb reboot
  6. Tap any edit field to show AOSP Keyboard.
  7. Confirm that the back button on the navbar works.
  8. Confirm that the IME switcher button on the navbar works.
Change-Id: I3e7e1f83554444131e2765dc159617bb9e2337c7