Customize screen behavior in landscape/portrait mode

[David]

Hi, 

    I am porting android to our TV Box product.

    When app request a portrait screen, the original Android behavior is rotate the screen, but our output is fixed position, the user can not use the device in this case.

    We change it to app use the center of the screen and leave the both side black, just like mirrorring iPhone to Apple TV.

    I study this topic for quite some time but still don't know where to start. Any hint?