You can set three invisible shortcut keys on bottom of screen (left, center and right). You can assign each key from more that 15 different available shortcuts.

Requirement : Rooted device. Lollipop

How to use
Open Virtual key app - Tap on Right/Left/Center button to enable corresponding virtual keys. (long press to disable)
To assign keys- scroll selected action from tab and tap on selected item to set (message will be displayed on successful assign on successful assign). You can close application after asg keys.
Now long press bottom corner/Center of screen to perform selected (assigned) action. See above screenshot for graphical representation of virtual keys. Long press on that are to execute assigned action.

Also there is option in menu to prevent aggressive killing of application if this vertual key gets killed automatically (happens with some samsung devices latest firmwares)

Each key have following options to assign
None
Browser
Calendar

Home
Menu
OK Google
RAM
Recent
Search
Sleep (Power button to turn off screen)
Screenshot
S-Voice
Sattelite view
Multiwindow
Battery
Kill foreground app