rtcp::App moved into own file and got Parse function

Review URL: https://codereview.webrtc.org/1437353003

Cr-Commit-Position: refs/heads/master@{#10688}

webrtc/modules/rtp_rtcp/source/rtcp_packet_unittest.cc

@@ -14,6 +14,7 @@
 #include "testing/gtest/include/gtest/gtest.h"
 
 #include "webrtc/modules/rtp_rtcp/source/rtcp_packet.h"
+#include "webrtc/modules/rtp_rtcp/source/rtcp_packet/app.h"
 #include "webrtc/test/rtcp_packet_parser.h"
 
 using ::testing::ElementsAre;