Settings: Restore functionality of reset in colour and gamma hardware tunables

Change http://review.cyanogenmod.org/#/c/50147/1 introduced a requirement that the user
manually change slider settings in colour and gamma controls before the values would write
to file, breaking the reset behaviour.

Change-Id: I5049631d2fa2a8d3553e8d56f49dc59770ecd6fb
(cherry picked from commit 3a36762ccdb12b765a7b671e8eeaec0adadd4a18)