bridge_softmix: Add additional old states for adding new source.

bridge_softmix: Add additional old states for adding new source.

There are three states that an old stream can be in to allow
becoming a source stream in a new stream:

1. Removed
2. Inactive
3. Sendonly

This change adds the two missing ones, inactive and sendonly,
so if a stream transitions from those to a state where they are
providing video to Asterisk we properly re-negotiate the other
participants.

ASTERISK-28944

Change-Id: Id8256b9b254b403411586284bbaedbf50452de01