Create //webrtc/api:libjingle_peerconnection_api + refactorings.

Create a new target //webrtc/api:libjingle_peerconnection_api and start moving
things into it. Move remaining parts of //webrtc/api:libjingle_peerconnection
to //webrtc/pc:libjingle_peerconnection.

Moved the RTCStatsCollectorCallback into its own header file, so that
PeerConnectionInterface can include that instead of pulling in
RTCStatsCollector and PeerConnection and everything.

Separated cricket::MediaType into its own header/source set, so that it
can be used in the api.

BUG=webrtc:5883

Review-Url: https://codereview.webrtc.org/2514883002
Cr-Commit-Position: refs/heads/master@{#16210}

webrtc/examples/BUILD.gn

@@ -505,7 +505,7 @@ if (is_linux || is_win) {
     }
     deps = [
       "//third_party/libyuv",
-      "//webrtc/api:libjingle_peerconnection",
+      "//webrtc/pc:libjingle_peerconnection",
       "//webrtc/system_wrappers:field_trial_default",
       "//webrtc/system_wrappers:metrics_default",
     ]

webrtc/examples/DEPS

@@ -6,4 +6,5 @@ include_rules = [
   "+webrtc/modules/audio_device",
   "+webrtc/modules/video_capture",
   "+webrtc/p2p",
+  "+webrtc/pc",
 ]