Relax rate control for last few frames

VBR rate control can become very noisy for the last few frames.
If there are a few bits to spare or a small overshoot then the
target rate and hence quantizer may start to fluctuate wildly.

This patch prevents further adjustment of the active Q limits for
the last few frames.

Patch also removes some redundant variables and makes one small bug fix.

Change-Id: Ic167831bec79acc9f0d7e4698bcc4bb188840c45