出现如下问题是因为flutter sdk中minSdkVersion是16，小于要求的最低版本19，
Manifest merger failed : uses-sdk:minSdkVersion 16 cannot be smaller than version 19 declared in library [:webview_flutter_android] E:\Demo\Flutter\flutter_android\build\webview_flutter_android\intermediates\merged_manifest\debug\AndroidManifest.xml as the library might be using APIs not available in 16
Suggestion: use a compatible library with a minSdk of at most 16,
or increase this project’s minSdk version to at least 19,
or use tools:overrideLibrary=“io.flutter.plugins.webviewflutter” to force usage (may lead to runtime failures)
打开flutter.gradle，找到minSdkVersion，修改版本为>=19就可以了。flutter.gradle路径如下（我的在D盘）
