Android P Volume Slider is Volume control Widget recently introduced in first developer preview of Android P.
Its a new vertical volume control to adjust volume in android P and onward updates.
Android P Volume Slider allows exactly the same Volume control widget for all android devices from android 4.1+ on wards.
No wait for official android P support for your device. install Android P Volume Slider app and have a similar experience for controlling volume for your phone.
App shows floating volume control like android P and its floating volume control that runs above all running apps and games, so without leaving your running app, you can easily increase or decrease volume of game or app.
At free it's worth trying out