Bias estimate of sound pressure level upwards

This patch introduced asymmetry in the loudness estimate
for the dynamic loudness correction. The effect is to
bias the estimated loudness towards the strongest sounds
heard in the last few seconds. This should fix the following
defects:

- reduce pumping treble associated with slow drum beats
- reduce the bass boost for sounds preceded by silence.

Change-Id: I5832ba7c43933ff4b71521ea4818aa9f7e3b4b65