首先给出正解：
其他网友的回答：
http://blog.sina.com.cn/s/blog_6cbd24890102x0ki.html
关键部分内容复制一下：
Note: The Run button builds an APK with testOnly="true", which means the APK can only be installed via adb (which Android Studio uses). If you want a debuggable APK that people can install without adb, select your debug variant and click Build > Build APK(s).
也就是说，只要在AndroidStudio3.0内，点击绿三角运行Run，跑出来的apk，一定是TEST_ONLY的。