We currently rely on the raw playback head position to check if any
data is pending in the AudioTrack (e.g. to know if the renderer is
still ready). We can use the value returned from getCurrentPositionUs
instead to align the "isReady" logic with the playback position logic.
This has the side effect that getPlaybackHeadPosition position is called
less often when the position is obtained via getTimestamp.
PiperOrigin-RevId: 514747613
