SDK / exoplayer

The `DashMediaSource` wrongly added an offset to the media times set
to the `MediaLoadData`. With this the `startTimeMS` and `endTimeMs`
don't represent the positions in the period but in the stream.

`DashMediaSource` was the only call site that was setting the offset
to a non-zero value. So if we are using 0 for the `DashMediaSource`
as well, the offset is redundant and we can remove it everywhere.

PiperOrigin-RevId: 520682026