audiodecoder: Take into account subclass PTS if provided
Submitted by Edward Hervey
Some decoder implementations operate with a different thread and know best
what output data correspond to which input data.
for those implementation (such as the androidmedia audio decoder), allow subclasses
to provide a timestamp for the decoder buffer.
The base class will stil do the various tolerance/drift/.. checks, but will use
the provided timestamp instead of the (potentially bogus) head frame value.