Add a new method for text gamma correction

To select the gamma correction method, adb shell setprop hwui.text_gamma_correction
with one of the following values:

lookup3
lookup
shader3
shader

See Properties.h for more information about these different methods.
You can also control gamma correction using the following properties:

hwui.text_gamma
hwui.text_gamma.black_threshold
hwui.text_gamma.white_threshold

Change-Id: I47970b804d2c590c37d3da5008db094241579e25