Added support for changing the volume of AudioTrack as discussed in BUG=webrtc:6533

This is a short term solution to change the volume of an AudioTrack until applyConstraints for MediaStreamTracks has been implemented.

This CL adds 1 new Java method & the relevant JNI file update:

AudioTrack.java:

public void setVolume(double volume);

BUG=webrtc:6533

Review-Url: https://codereview.webrtc.org/2710683009
Cr-Commit-Position: refs/heads/master@{#17682}

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

@@ -15,4 +15,13 @@ public class AudioTrack extends MediaStreamTrack {
   public AudioTrack(long nativeTrack) {
     super(nativeTrack);
   }
+
+  /** Sets the volume for the underlying MediaSource. Volume is a gain value in the range
+   *  0 to 10.
+   */
+  public void setVolume(double volume) {
+    nativeSetVolume(super.nativeTrack, volume);
+  }
+
+  private static native void nativeSetVolume(long nativeTrack, double volume);
 }