minSdkVersion 16 cannot be small

minSdkVersion 16 cannot be smaller than version 19 declared in library 

解决方法，对应的把16改成19：

本地的flutter路径

1、flutter_sdk/packages/flutter_tools/templates/module/android/host_app_common/app.tmpl/build.gradle.tmpl

2、flutter>packages>flatter_tools>gradle>flatter更改flutter.minSdkVersion