res_pjsip: Add utils for checking media types

Added two new functions to assist checking media types...

* ast_sip_are_media_types_equal compares two pjsip_media_types.
* ast_sip_is_media_type_in tests if one media type is in a list
  of others.

Added static definitions for commonly used media types to
res_pjsip.h.

Changed several modules to use the new functions and static
definitions.

ASTERISK_29813
(not ready to close)

Change-Id: Ief77675235bd3bf00a6b095d4673fd878d0801b9