MD coding in ISM inactive frames

In ISM format, the transmission of metadata forces the core-coder coding always to active frames as there is currently no mechanism to transmit MD in inactive frames. Consequently, the total_brate changes too much (it actually has spikes) as MD are inserted into inactive frames at least every 0.5 sec for FEC reasons.

A fix will be provided that enables the use of low-bitrate mode with MD in inactive frames. This new logic will have some impact on the VAD signaling and coding of inactive frames with NULL metadata.