Add overlay for navigation bar buttons

Squashed commit of the following, from togari

commit d581b32c6133d5f00dd008994d558efcc67fdba7
Author: Jiangyi <sam.andrew.jiang@gmail.com>
Date:   Thu Aug 21 12:13:48 2014 -0400

    togari: Fix up navigation bar layout

    Instead of using two dummy views to centre the keys, have the
    centre LinearLayout use margins instead.

    Change-Id: Ic7cdd171c0480048e73a7f1da49dc2e5514369e6

commit 7bf3108706f84c0938a48bc5586ed9279bb9ee17
Author: Chirayu Desai <cdesai@cyanogenmod.org>
Date:   Sat Jul 5 19:22:03 2014 +0530

    overlay: SystemUI: add left and right virtual buttons while typing

    Change-Id: Iea446443d33ff305765e47c86523b619676116ac

commit 5e2cd59fc1d743460e36a48ae05aec6b5e56c7db
Author: Chirayu Desai <cdesai@cyanogenmod.org>
Date:   Wed Jul 2 12:40:04 2014 +0530

    Fix uneven spacing of navigation bar buttons in landscape

    * Add two dummy views to the left and right corners to
      bring the buttons nearer to the center
    * This makes it easier to hit the back and recents key,
      and makes it identical to what the Sony firmware has.

    Same as commit 'd2af20ff1077b2edbc85160dde94db809dbf3643',
    but for landscape

    Change-Id: Ib4c531d587e56db8878b00a5bd2e2a738692b622

commit d2af20ff1077b2edbc85160dde94db809dbf3643
Author: Chirayu Desai <cdesai@cyanogenmod.org>
Date:   Mon Mar 3 19:36:50 2014 +0530

    Fix uneven spacing of navigation bar buttons

    * Add two dummy views to the left and right corners to
      bring the buttons nearer to the center
    * This makes it easier to hit the back and recents key,
      and makes it identical to what the Sony firmware has.

    It's the litte things that matter :)

    Change-Id: I180ed2cf1370139f0bbadad963881b20f269e76d

Change-Id: I8475bc188abe6ce0d2f5d12dd05547f4e69824ef