Never set minSdk and targetSdk in manifest.

android/src/main/AndroidManifest.xml

@@ -1,7 +1,5 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
-          package="com.flutter_webview_plugin"
-          android:versionCode="1"
-          android:versionName="0.0.1">
+          package="com.flutter_webview_plugin">
 
-    <uses-sdk android:minSdkVersion="16" android:targetSdkVersion="21"/>
+    <uses-sdk android:minSdkVersion="16" android:targetSdkVersion="27"/>
 </manifest>