This refactoring improves user experience, fix an issue and simplify code.

- less exposed "internal" stuff (RTP sequence value)
- don't expose internal business logic to upper layers.
- remove uneeded full media restart: only media_encoder is restarted
- fixed non protected shared access to codec isRunning value causing
  a race condition on access and good design practices violation
  (tight coupling between unrelated classes).

Refs #77483

Change-Id: Idb1e3940601ff0d483e7721f627a0c8dd766f844