media_encoder: add setPacketLoss method

Add a setPacketLoss method to enable setting a new packet loss
estimation in the codec.
This method follows the model of setBitrate and requires the
implementation of getCurrentAudioAVCtx and isDynPacketLossSupported,
which are also included in this patch.

Issue: #5157
Change-Id: I62f5246df1df0dc0867649299c6bbf6c12db43a2