SDK / exoplayer

There are a couple of problems with how positions in PlaybackInfo are set at
the moment:
1. PositionUs isn't allowed to be C.TIME_UNSET. This is prevented by always
resolving to the current default position if needed.
2. In some places a window position was used as a period position. Use correct
position resolution procedure.
3. When creating a placeholder position to restart playback, we used the first
period in a window, not the one where the default position is.
4. The start position for ads was in some cases set to 0 without checking
the ad resume position.

PiperOrigin-RevId: 290749042