Move fade-in of buffer into AudioTrackAudioOutput, as not every output has same requirement as AudioTrack

[REQUIRED] Use case description

I'm using AudioOutput based on libusb, so Android audio stack is uninvolved. DefaultAudioSink adds fade-in to every beginning playback after AudioSink.flush() - see also https://github.com/androidx/media/commit/5e3dcea1bfffff0f641d34829d6965a89c791b68 -, but this isn't needed in this case as it's not going into AudioTrack, and volume fade is already accounted for.

Proposed solution

Move the maybeRampUpVolume() logic into AudioTrackAudioOutput, where it doesn't affect custom audio outputs. The commit and logic was explicitly designed around where AudioTrack does or doesn't fade, and other outputs should add this if needed, or don't add it if not.

Happy to provide a PR for this if this solution makes sense!

Alternatives considered

Add a on/off setting to DefaultAudioSink for this fade which I change in lockstep with output provider.