Add Use AndroidX property

Ref: https://developer.android.com/jetpack/androidx#using_androidx

android/gradle.properties

 ReactNativeWebView_compileSdkVersion=28
 ReactNativeWebView_buildToolsVersion=28.0.3
 ReactNativeWebView_targetSdkVersion=28
-ReactNativeWebView_supportLibVersion=28.0.0
+ReactNativeWebView_supportLibVersion=28.0.0
+android.useAndroidX=true