support specify audio encode bitrateMode interface

[bdrtc]

fixes #649

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[dalecurtis]

Do we want a config level bitrate mode knob like we have for video instead?

[Djuffin]

I agree with @dalecurtis, bitrate mode seems like a very generic knob that will be used for many codecs with the same meaning. So I think it belongs in AudioEncoderConfig

[bdrtc]

AudioEncoderConfig

Thanks feedback, I considered add this to AudioEncoderConfig before commit, but for audio codec, not all codecs support vbr mode, For example ulaw/alaw, what will happen if user specify a bitrate mode to AudioEncoderConfig that current codec does't supported ?

[Djuffin]

what will happen if user specify a bitrate mode to AudioEncoderConfig that current codec does't supported ?

AudioEncoder.isConfigSupported() will return false.

[bdrtc]

Do we want a config level bitrate mode knob like we have for video instead?

I will move it to AudioEncoderConfig and add a Note, Thanks.

[bdrtc]

Dear editors, any more comments about this ? thanks.

[Djuffin]

LGTM @tguilbert-google please take a look

[tguilbert-google]

Also LGTM!

[bdrtc]

Also LGTM!

Thanks. If there's any questions please let me know when you approve this.