Remove extend touchable region for AOSP keyboard

Remove EXTENDED_TOUCHABLE_REGION_HEIGHT from LatinIME#onComputeInsets
to prevent keyboard touch region covered navigation bar
when in split-window mode with display density < 240 case.

Fix: 134893742
Test: manual as below steps:
1) Set window density as 240 with "adb shell wm density 240"
2) Launch a app (i.e. Messages) from recents activity, set as split-screen mode.
3) Tap Search bar to show IME keyboard.
4) Press home / back / recents key if it works, expect it works.

Change-Id: I596b7276041fecc50d2bc095c7e51664f632368d