Stop encoding/decoding presentation time as part of the message.
What's actually in emsg boxes is a presentation time delta,
which is why it's only 32 bits, and hence why it doesn't handle
large absolute timestamps. We were using this field to hold
absolute timestamps only for the purpose of passing presentation
times from DashManifestParser.parseEvent back to the calling
method. After this change, we return Pair<Long, EventMessage>
instead.
Issue: #5490
PiperOrigin-RevId: 233561731
c66d26b601