device_config: Enable maching learning back gesture

This enables machine learning-assisted back gestures when using gesture
navigation. It uses TensorFlow Lite for inference (via our custom
provider in SystemUI), similar to Google's Pixel implementation.

overlay/packages/apps/SimpleDeviceConfig/res/values/config.xml

@@ -33,6 +33,11 @@
         <!-- Cached app freezer on supported devices -->
         <item>activity_manager_native_boot/use_freezer=true</item>
 
+        <!-- Machine learning back gesture (requires TensorFlow model) -->
+        <item>systemui/use_back_gesture_ml_model=true</item>
+        <item>systemui/back_gesture_ml_model_name=backgesture</item>
+        <item>systemui/back_gesture_ml_model_threshold=0.9</item>
+
         <!--
             Disable the TF input classifier entirely to save CPU and fix log
             spam when we have the touch heatmap video interface disabled. This