Extend Android SDK to enable adding custom PortAllocator

This patch exposes the network_thread so that
a custom PortAllocator can use it instead of e.g
creating own thread.

Bug: webrtc:8640
Change-Id: I705629e4f1a4d0a4fed7d53a774ba9564ba076fe
Reviewed-on: https://webrtc-review.googlesource.com/39925
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Commit-Queue: Jonas Oreland <jonaso@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21643}

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

@@ -347,6 +347,11 @@ public class PeerConnectionFactory {
     return nativeGetNativePeerConnectionFactory(nativeFactory);
   }
 
+  /** Returns a pointer to the native OwnedFactoryAndThreads object */
+  public long getNativeOwnedFactoryAndThreads() {
+    return nativeFactory;
+  }
+
   private static void printStackTrace(Thread thread, String threadName) {
     if (thread != null) {
       StackTraceElement[] stackTraces = thread.getStackTrace();