Add traces to BackGestureTFClassifier#loadVocab

This seems to take a lot of time, and we think it happens more often than needed. With those traces we will be able to easily spot it in perfetto recordings.

Bug: 197515205
Test: Manual
Change-Id: Iaa3d14ebf789d7d2082de121e58ca443fc4122c1

packages/SystemUI/src/com/android/systemui/navigationbar/gestural/EdgeBackGestureHandler.java

@@ -36,6 +36,7 @@ import android.os.Looper;
 import android.os.RemoteException;
 import android.os.SystemClock;
 import android.os.SystemProperties;
+import android.os.Trace;
 import android.provider.DeviceConfig;
 import android.util.DisplayMetrics;
 import android.util.Log;
@@ -576,7 +577,9 @@ public class EdgeBackGestureHandler extends CurrentUserTracker
             mMLModelThreshold = DeviceConfig.getFloat(DeviceConfig.NAMESPACE_SYSTEMUI,
                     SystemUiDeviceConfigFlags.BACK_GESTURE_ML_MODEL_THRESHOLD, 0.9f);
             if (mBackGestureTfClassifierProvider.isActive()) {
+                Trace.beginSection("EdgeBackGestureHandler#loadVocab");
                 mVocab = mBackGestureTfClassifierProvider.loadVocab(mContext.getAssets());
+                Trace.endSection();
                 mUseMLModel = true;
                 return;
             }