Add methods to ExoPlayer for setting/getting the playback speed, using
SonicAudioProcessor.
Remove PlaybackParams support, as the AudioTrack timestamp does not work
reliably on Marshmallow. The platform also uses Sonic and performance
should be comparable between the Java and native versions on recent Android
runtimes.
In a later change, SonicAudioProcessor will be made public so it can
be used in conjunction with other processors.
Issue: #26
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Created by MOE: https://github.com/google/moe
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