Use AndroidX

app/src/main/java/com/hippo/drawable/BitmapPool.java

@@ -17,9 +17,8 @@
 package com.hippo.drawable;
 
 import android.graphics.Bitmap;
-import android.support.annotation.Nullable;
 import android.util.Log;
-
+import androidx.annotation.Nullable;
 import java.lang.ref.WeakReference;
 import java.util.Iterator;
 import java.util.LinkedHashSet;