Fix lint error

Author: yuyakaido

sample/src/main/java/com/yuyakaido/android/cardstackview/sample/MainActivity.kt

@@ -2,7 +2,6 @@ package com.yuyakaido.android.cardstackview.sample
 
 import android.os.Bundle
 import android.util.Log
-import android.view.Gravity
 import android.view.View
 import android.view.animation.AccelerateInterpolator
 import android.view.animation.DecelerateInterpolator
@@ -11,6 +10,7 @@ import android.widget.TextView
 import androidx.appcompat.app.ActionBarDrawerToggle
 import androidx.appcompat.app.AppCompatActivity
 import androidx.appcompat.widget.Toolbar
+import androidx.core.view.GravityCompat
 import androidx.drawerlayout.widget.DrawerLayout
 import androidx.recyclerview.widget.DefaultItemAnimator
 import androidx.recyclerview.widget.DiffUtil
@@ -34,7 +34,7 @@ class MainActivity : AppCompatActivity(), CardStackListener {
     }
 
     override fun onBackPressed() {
-        if (drawerLayout.isDrawerOpen(Gravity.START)) {
+        if (drawerLayout.isDrawerOpen(GravityCompat.START)) {
             drawerLayout.closeDrawers()
         } else {
             super.onBackPressed()