Delete size limitation for ContrastColorUtil.isGrayScale() method.

loss of performance is tiny because, image will be resized at ImageUtil.isGrayscale() method.

Test: situation that one notification is sended to other resoltion devices, ex) using DEX mode,
Test: notification can be inflated at different resolution context.
Test: and app can have notification icon that is made dynamically.
Test: (it doesn't have resource for each resolution. ex: calendar notice icon with number)
Test: at this moment, judgement for grayscale can be differed from each device.
Change-Id: I2755c56869aba58287137f683a81ec4ddfd3d7c1