Improve background noise estimation for CELT DTX

We now update the background noise estimate even in frames classified
as transient. It shouldn't be a problem because we're using min
statistics. Also, it avoids problems when update frames get
missclassified as transient.

In addition, we now use the duration of losses rather than the
number of lost packets to make decisions. That should make
PLC/DTX behaviour more consistent across frame sizes.