systemui: add brightness slider 2/2

commit d1b590bd24b7e8b7f443f44daedaafc14df195db
Author: PrimeDirective <activethrasher00@gmail.com>
Date:   Sun Sep 1 00:56:25 2013 -0400

    fixup statusbar brightness slider

    now is a singleton to reduce leakage and observer weight also

    Change-Id: I55fc9a2e1de81d38147788d3fe90d410ee9ffd03

commit 683492a9baa55c78967ce9e009f06b90c0ad30e7
Author: Roman Birg <romanbirg@gmail.com>
Date:   Wed Dec 19 23:08:50 2012 -0600

    SystemUI: add statusbar slider preference

    Change-Id: Ibad11cd0495aa5fc2aa31e6f6c1e1cf85b8a582b
    Signed-off-by: Roman Birg <romanbirg@gmail.com>

ps2: make whitespace LESS SHITTY
ps3: add a missing handler... g2g MY ASS

Change-Id: I9744fcfd4452f73e752211945f78308e57a1604a