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.
- 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