Fix PTS produced by the default opus/flac encoders

The default OPUS and FLAC encoders on Android rewrite the input PTS so
that they exactly match the number of samples.

As a consequence:
 - audio clock drift is not compensated
 - implicit silences (without packets) are ignored

To work around this behavior, generate new PTS based on the current time
(after encoding) and the packet duration.

PR #5870 <https://github.com/Genymobile/scrcpy/pull/5870>