Show touchpad entry when input device is detected as touchpad.

1. Remove the redundant part about active ime.
2. Add a NPE detector to fix bug.
3. Check if input device is detected as touchpad.

Bug: 247080509
Bug: 267092726
Test: manual
Change-Id: I40640e42abd471efde6d1f9166b8999243499c62

src/com/android/settings/inputmethod/TouchGesturesButtonPreferenceController.java

@@ -63,7 +63,9 @@ public class TouchGesturesButtonPreferenceController extends BasePreferenceContr
 
     @Override
     public int getAvailabilityStatus() {
-        return AVAILABLE;
+        boolean touchGestureDeveloperMode = FeatureFlagUtils
+                .isEnabled(mContext, FeatureFlagUtils.SETTINGS_NEW_KEYBOARD_TRACKPAD_GESTURE);
+        return touchGestureDeveloperMode ? AVAILABLE : CONDITIONALLY_UNAVAILABLE;
     }
 
     private void showTouchpadGestureEducation() {