Values now carried over frame to frame.
Change to algorithm for decreasing threshold after
a hit and to max threshold (now based on speed)

Removed some old commented out code relating to
VP8 adaptive thresholds.

The impact of these changes tested on Akiyo (50 frames)
and measured in terms of unit rd hits is as follows:

Speed 0 84.36 -> 84.67
Speed 1 29.48 -> 22.22
Speed 2 11.76 -> 8.21
Speed 3 12.32 -> 7.21

Encode speed impact is broadly in line with these.

Change-Id: I5b886efee3077a11553fa950d796fd6d00c8cb19