sharedpreferences not work on android and showing error.

when i run flutter run apk --release
below error shows
```console
D:\flutter\.pub-cache\hosted\pub.dartlang.org\shared_preferences-0.5.12+4\android\src\main\java\io\flutter\plugins\sharedpreferences\MethodCallHandlerImpl.java:9: warning: [deprecation
] AsyncTask in android.os has been deprecated
import android.os.AsyncTask
```