Add new PeerConnection APIs to the Java SDK.

This adds wrappers to the following native APIs:
- SdpSemantics enum added to the RTCConfiguration
- RtpTransceiver
- PeerConnection.addTrack
- PeerConnection.removeTrack
- PeerConnection.addTransceiver
- PeerConnection.getTransceivers
These APIs are used with the new Unified Plan semantics.

Bug: webrtc:8869
Change-Id: I19443f3ff7ffc91a139ad8276331f09e57cec554
Reviewed-on: https://webrtc-review.googlesource.com/57800
Commit-Queue: Seth Hampson <shampson@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22317}

sdk/android/api/org/webrtc/MediaStream.java

@@ -88,13 +88,19 @@ public class MediaStream {
     JniCommon.nativeReleaseRef(nativeStream);
   }
 
+  // TODO(shampson): Remove this when downstreams have moved to using id() instead.
+  @Deprecated
   public String label() {
-    return nativeGetLabel(nativeStream);
+    return nativeGetId(nativeStream);
+  }
+
+  public String getId() {
+    return nativeGetId(nativeStream);
   }
 
   @Override
   public String toString() {
-    return "[" + label() + ":A=" + audioTracks.size() + ":V=" + videoTracks.size() + "]";
+    return "[" + getId() + ":A=" + audioTracks.size() + ":V=" + videoTracks.size() + "]";
   }
 
   @CalledByNative
@@ -137,5 +143,5 @@ public class MediaStream {
       long stream, long nativeVideoTrack);
   private static native boolean nativeRemoveAudioTrack(long stream, long nativeAudioTrack);
   private static native boolean nativeRemoveVideoTrack(long stream, long nativeVideoTrack);
-  private static native String nativeGetLabel(long stream);
+  private static native String nativeGetId(long stream);
 }