Turn "allow_screen_content_tools" on when the source video has many blocks
with only few different colors. The automatic detection is enabled by
defualt (or with command line flag "--tune-content=default"). With
"--tune-content=screen", the screen content tools are always turned on.

On the screen_content test set, the "default" setting is less than 0.3%
worse than the "screen" setting on keyframe encoding.

Change-Id: Iac7ab8952c96531d1fae84da1823291f5987519c