Fixed waveform for audios under 1 second not being extractable on Android

[mschudt]

Fixed divison by zero due to duration value being rounded to 0 for audios under 1 second length. I changed the variable from duration (in seconds) to durationMillis which fixes an exception that was thrown.

Offset argument contained a NaN value.
'dart:ui/painting.dart':
Failed assertion: line 39 pos 10: '<optimized out>'