Create an AudioProcessor implementation for ReplayGain.
Now that ExoPlayer handles AudioFocus, the ReplayGain implementation
would conflict with the changes that ducking would make to the volume.
To fix this, migrate the ReplayGain implementation to a dedicated
audio processor. This not only resolves this system, but also opens the
door for positive ReplayGain values in the future. Currently however,
our method for modifying the bitstream results in popping with values
above the reference volume, so some more work must be done in that
regard.
b4abad26cd