overlay: Update display brightness setup to AOSP's

This is a squash of the following commits:

commit 6cc669975b0afbbcc5de0e591fea039c87fb7e53
Author: Adam Farden <adam@farden.cz>
Date:   Sun May 1 13:08:57 2016 +0200

    overlay: improve the brightness curve

    The old curve was edited manually to appear linear on a log graph,
    but this ramps up the brightness too easily. The new curve is now
    simply `pow(brightness, 2)/8`

    In a dark environment the display no longer burns your retina.

Signed-off-by: Adam Farden <adam@farden.cz>

commit a0d889245e064525f87684a70caeeae583e9e538
Author: Alin Jerpelea <alin.jerpelea@sonymobile.com>
Date:   Sun Feb 7 11:15:02 2016 +0100

    update backlight levels

    device has a lower light backlight and we have to boost levels
    to match the other devices

Signed-off-by: Alin Jerpelea <alin.jerpelea@sonymobile.com>

commit 5f04ea3c971df5fb02fb47c0b2844a8b73cc50f1
Author: Humberto Borba <humberos@gmail.com>
Date:   Sat Dec 5 21:48:51 2015 -0200

    ivy: overlay: Add LCD brightness setup

    This patch introduces LCD brightness values moved
    from kitakami common overlay definitions.

    Change-Id: I5c2d82257374cea7b7b4ad63607cbe87e280919e
Signed-off-by: Humberto Borba <humberos@gmail.com>

Change-Id: I54f123334c85ffbbd7cf96f7d668335cdc6ddf3b