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}