Installation failed with message Failed to establish session.
It is possible that this issue is resolved by uninstalling an txisting version fo the apk if it is present, and then re-installing.
WARNING:Uninstalling will remove the application data !
Do you want to uninstall the existing application?

解决方式（不一定适合你，反正解决了）
取消instant run （在此之前你也可以尝试clean project尝试下）

Disable Instant Run
Settings>Build, Execution, Deployment>Instant Run

虽说Instant Run功能有多多多强大，但是很多坑的，比如有时会你修改的代码不执行等等，慎用！