Enable audio volume control on Android.

Enable audio volume control on Android.

This is done by setting stream to VOICE which is the only stream type hooking up volume control automatically.
It's required to use matching audio mode and stream type. Otherwise, the volume control doesn't work. That means we have to use same mode
for both WebAudio and WebRTC. This leads to the change in audio_manager_android.cc which sets audio to communication mode so that OpenSL
stream can use VOICE stream type to adjust volume.

Contributed by leozwang@chromium.org (original patch https://chromiumcodereview.appspot.com/12806009/)

TBR=fischman@chromium.org

BUG=222393, 243506
TEST=run https://apprtc.appspot.com/?debug=loopback on Google Android devices, e.g., Galaxy Nexus, Nexus4. The volume can be changed with the volume key. Also run some WebAudio test samples, e.g., https://developer.cdn.mozilla.net/media/uploads/demos/b/o/boblemarin/bf84de2482b5c2e1a869089fad3de56e/3eat5urfin9_1362006031_demo_package/index.html.

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=207921

[wjia(left Chromium), 2013-06-20 20:43:17]

Ami,

PTAL. This CL is essentially same as
https://chromiumcodereview.appspot.com/12806009/, except that the description is
new.

[wjia(left Chromium), 2013-06-21 21:16:06]

Committed patchset #4 manually as r207921 (presubmit successful).