Always let the encoder decide if "allow_screen_content_tools" should be set.

-The screen content detector(is_screen_content()) works fairly well.

-Keeping the config control may lead to mismatches. For example, user
may encode the 1st frame with the flag on, then turn the flag off for
the 2nd frame via config control. On the decoder side, the flag is on
for both frames, because the flag is only transmitted in the header of
all-intra frame, i.e. the 1st frame in this example.

BUT=aomedia:656

Change-Id: I0652fe2b9f8df19bfbf7ed7d3513205c80ea9c61