demux: ts: flag corruption on packet loss instead of discontinuity

Simple packet loss is not a real discontinuity, but a corruption. Corruption is a hint, and shouldn't alter pipeline much than from a silent corruption. It is also up to the aggregation layer to validate and deal with the effects of corruption.

discontinuity as flag has lots of unwanted side effects, flushing and resyncing too hard. Light packet loss can end up with unwatchable content as a result.

As for now, corruption flag is dealt in a same way in packetizers and decoders. Readying the needed changes for #27571.