Settings: Use meaningful titles for gamma controls

There is good anecdotal evidence that the values we modify relate
to specific modifications (contrast, brightness, saturation and the
like) of the display, this change allows us to define descriptive
headings.

Change-Id: I2935924fd86f13e95b919e7b7c7a0bc931ac50bb

Conflicts:
	res/values/cm_arrays.xml