Add color mode enums

The android_color_mode_t defines the color modes supported
by a display. A display can support multiple different
color modes.

sRGB mode and AdobeRGB are deliberately placed at 7 and 8
so that they match mode ID's used on prior devices.

bug: http://b/27926012
Change-Id: I9ab41b56ec0b495c02539665da5be158ffddf843