[rtp_rtcp] rtcp::Empty moved into own file and renamed to CompoundPacket on the way

Class renamed to indicated use of the rtcp::Empty class: it can only be used as container for other rtcp packets.
All tests that use Append function moved from rtcp_packet_unittest, even if they did not use Empty class.
This is because there is plan to make RtcpPacket class lighter by moving Append functionality to CompoundPacket class.

BUG=webrtc:5260
R=åsapersson

Review URL: https://codereview.webrtc.org/1582503002

Cr-Commit-Position: refs/heads/master@{#11234}

webrtc/modules/rtp_rtcp/source/rtcp_packet.cc

@@ -390,17 +390,6 @@ void RtcpPacket::CreateHeader(
   AssignUWord16(buffer, pos, length);
 }
 
-bool Empty::Create(uint8_t* packet,
-                   size_t* index,
-                   size_t max_length,
-                   RtcpPacket::PacketReadyCallback* callback) const {
-  return true;
-}
-
-size_t Empty::BlockLength() const {
-  return 0;
-}
-
 bool SenderReport::Create(uint8_t* packet,
                           size_t* index,
                           size_t max_length,