Update the implementation of early media support

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