Use default button backlight brightnessk from framework overlay

Change-Id: Ib0361359a29e8030682707bf42c9fe04f780b103
Settings: QS performance profiles tile

Requires http://review.cyanogenmod.org/#/c/51776/

Patchset 2: Fix not null comparation
Patchset 3: Added suggestions
Patchset 4: Fix string
Patchset 5: Rebased
Patchset 6: Rebased
            Move setCurrentValue() inside mPerfProfilePref null check

Change-Id: I578d84002b00e0a7c58cdcaf2a3735bf6bbfdabf
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>
Merge "Settings: QS performance profiles tile" into cm-10.2
Merge branch 'cm-10.2' into martincz