Power on/off/reboot Android phone without using power button

Works without rooting the phone.

Power Off and Reboot

In Google Play Store, install app Button Mapper: Remap your keys. Remap one of the volume key to Power Dialogue. You can set Up Volume Long Press to bring up Power Dialogue.

Power On 

It may not work with every phone. Plug in power cable to charge it. When it show a simple charging (flash) icon on screen but not yet showing animated charging picture with percentage, unplug the cable. Wait a while, the phone will boot.

If your phone supports wireless charging, simply put it on charger. It will boot the phone right away.