解决方法：去掉“Enable Instant run to host swap code/resource changes on deploy(default enabled)”的勾选项
Settings→Build→Instant Run:
1：Enable Instant Run to hot swap code/resource changes on deploy (default enabled)
//启用即时运行 热交换代码/资源变化 部署(默认启用)
2：Restart activity on code changes
//当代码变动时重启
3：Show toasts in the running app when changes are applied
//当代码变动应用时弹出toast
4：show Instant Run status notifications
//在notification里显示instant Run 的状态
当再次勾选后出现了新问题
The APK file D:\Android\del\MaterialDemo\app\build\outputs\apk\app-debug.apk does not exist on disk.
Error while Installing APK
解决方案：build-->build apk