Why don't the OS buttons lower when the game is playing?

Hello,

I have just play tested my game on my HTC M8 (via USB cable debugging) and I have realised that the operating system's buttons do not lower. It is killing valuable game space that could be used. Has Unity made a function or is there a solution for this?

I am using Unity 4.7 (yes, it's old I know) with screen size 360 X 640

Here are the buttons that I mean:

Thanks