Are you wanting to know because you prefer to know the coding behind it or just that you would prefer to be able to control the position? If you want the easier way, it would be to install the Xposed Framework and a module like Gravity Box. From what I have read, it's a huge hassle to edit everything needed to get this to work right. You might find some helpful info here: http://android-developers.blogspot.c...nu-button.html
about how this stuff is implemented in code. Unless you really just wanna figure it out for the sake of figuring it out and learning, go with the Xposed mod. Alternative, you could look into http://forum.xda-developers.com/show....php?t=2270198
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