feat(android): Migrate to Android X (#520)

BREAKING CHANGE:
If you override compileSdkVersion<28  it won't work anymore.

android/build.gradle

   }
 }
 
-def support_version = getExtOrDefault('supportLibVersion')
 def kotlin_version = getExtOrDefault('kotlinVersion')
 
 dependencies {
   //noinspection GradleDynamicVersion
   api 'com.facebook.react:react-native:+'
   implementation "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
-  implementation "com.android.support:appcompat-v7:$support_version"
 }

android/gradle.properties

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

android/src/main/java/com/reactnativecommunity/webview/RNCWebViewFileProvider.java

 package com.reactnativecommunity.webview;
 
-import android.support.v4.content.FileProvider;
+import androidx.core.content.FileProvider;
 
 /**
  * Providing a custom {@code FileProvider} prevents manifest {@code <provider>} name collisions.

android/src/main/java/com/reactnativecommunity/webview/RNCWebViewModule.java

 import android.os.Environment;
 import android.os.Parcelable;
 import android.provider.MediaStore;
-import android.support.annotation.RequiresApi;
-import android.support.v4.content.ContextCompat;
-import android.support.v4.content.FileProvider;
+import androidx.annotation.RequiresApi;
+import androidx.core.content.ContextCompat;
+import androidx.core.content.FileProvider;
 import android.util.Log;
 import android.webkit.MimeTypeMap;
 import android.webkit.ValueCallback;