Discontinuities are only signalled on streams that have been identified, i.e.

those that have a queue. This ensures that the player doesn't observe discontinuities
that don't match up across streams.

Also, make sure output buffers arriving from the decoder to be rendered are sent
back to the decoder if we started flushing.

Finally, don't parse TS packets for streams we don't support. And don't allocate
memory for them.

Change-Id: I708e0de4cba8110a62e4c8ceb1e5702430d5d2bb