Android Feature add multi-window (split-window)

[dapgo]

[Description]:

In android 7 (samsung) apps supporting multi-window allow to display several apps on screen at sametime.

Ghost commander is an example of FOSS app which supports multi-window.

[Affected versions]:

1.0android-c2791

[Steps to reproduce]:

In Samsung android v7,keep back button for a while, then a vertical menu bar is displayed with multi window apps, after clicking 'edit' installed apps supporing multi window are displayed.