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}
diff --git a/AUTHORS b/AUTHORS
index 126a138..e13033e 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -12,6 +12,7 @@
 Christophe Dumez <ch.dumez@samsung.com>
 Cody Barnes <conceptgenesis@gmail.com>
 Colin Plumb
+Dax Booysen <dax@younow.com>
 Dmitry Lizin <sdkdimon@gmail.com>
 Eric Rescorla, RTFM Inc. <ekr@rtfm.com>
 Frederik Riedel, Frogg GmbH <frederik.riedel@frogg.io>