avdevice: use av_gettime_relative() for elapsed time calculations

avdevice: use av_gettime_relative() for elapsed time calculations

av_gettime_relative() is using the monotonic clock therefore more suitable for
elapsed time calculations. Packet timestamps are still kept absolute, although
that should be configurable in the future.

Related to ticket #9089.

Signed-off-by: Marton Balint <cus@passwd.hu>