Update the implementation of early media support, REF 15059

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