Visibility Checker update for iOS 12.
Visibility Checker update for iOS 12.

* Change logic for an element being different from a pixel by pixel comparison to a 2 point threshold per color channel.
* To prevent issues with the corner radius, we now remove it completely, perform the visibility check and then then reset it.

google/EarlGrey,google/EarlGrey,google/EarlGrey,google/EarlGrey,google/EarlGrey