Instead using only a fixed threshold, this commit adapts the threshold
for color sensitivity decision to luma signal energy: chroma channel's
sse is at least 1/6 of that in luma for color sensitivity flag to be
set to active.

This recoups a large portion of the speed loss due to accounting for
chroma component costs in RTC mode decision.

Change-Id: Ie01f747f6037dba6a1d1ed3e10b71a0ef1abc42c