报错原因：

* What went wrong:

Execution failed for task ':app:processDebugManifest'.

> Manifest merger failed : uses-sdk:minSdkVersion 19 cannot be smaller than version 21 declared in library [:facerecognitionlibrary-debug:] C:\Users\user\.gradle\caches\transforms-1\files-1.1\facerecognitionlibrary-debug.aar\7a3a4b635de9129fbfbd1793ee59e58f\AndroidManifest.xml as the library might be using APIs not available in 19

        Suggestion: use a compatible library with a minSdk of at most 19,

                or increase this project's minSdk version to at least 21,

                or use tools:overrideLibrary="ch.zhaw.facerecognitionlibrary" to force usage (may lead to runtime failures)

解决办法：

修改/platforms/android/build.gradle中defaultMinSdkVersion=21或者修改arr中build.gradle文件minSdkVersion为21（需要在原项目中修改，然后重新rebuild porject）