call: move media related API in dedicated header section

Media API are in refactoring.
To help to this huge task this patch moves all media related
API declaration from Call class header into a dedicated section.
It's help maintainance and new insertion, then it help to detect
what is wrong with current media design.

Change-Id: I4f65436112337b4a44eea37680d56e0fb4f5696d