lottie/audio: fix the audio offset of trimmed layers

The offset subtracted the layer in-point,
which cancels out the start time. A trimmed audio
at its head then always played from the
beginning of the source, so the trimmed
part was heard and the tail was cut off instead.

Use the remapped layer time directly,
keeping the source position that `st` defines and remaining
correct under time stretch and remapping.