The following changes are made in this commit since only backward (from UAS to UAC)
early media needs to be supported as per RFC5009.

* Always include "P-Early-Media: supported" in a transmitted INVITE regardless the
  configuration
* Always include "P-Early-Media: sendonly" in PRACK to an incoming 18x response
  which includs "P-Early-Media" regardless the configuration
* Don't include "P-Early-Media" header in any responses to an incoming INVITE
* Always play early media if any in an outgoing call