Throw IllegalStateException if native objects are used after dispose.

This makes it easier to debug issues related to double dispose /
use after dispose.

Bug: webrtc:7566, webrtc:8297
Change-Id: I07429b2b794deabb62b5f3ea1cf92eea6f66a149
Reviewed-on: https://webrtc-review.googlesource.com/102540
Commit-Queue: Sami Kalliomäki <sakal@webrtc.org>
Reviewed-by: Paulina Hensman <phensman@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24894}

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

@@ -18,4 +18,9 @@ public class AudioSource extends MediaSource {
   public AudioSource(long nativeSource) {
     super(nativeSource);
   }
+
+  /** Returns a pointer to webrtc::AudioSourceInterface. */
+  long getNativeAudioSource() {
+    return getNativeMediaSource();
+  }
 }