The value returned by `player.getContentPosition()` is calculated
in the timeline based on the position of the first period in the
window. In a single period live stream this position is advanced
when the live window advances on timeline refresh.
This calculation has produced slightly varying values below 1000
us which are likely caused by us/ms truncations for public API
values that we use in the IMASSAIMediaSource.
However, `AdGroup.timeUs` is the (recorded) content position at the
moment when the first ad of the an ad group has been inserted. While
playing an ad, we can always use this value instead of
`getContentPosition()` to not require recalculation.
#minor-release
PiperOrigin-RevId: 515093177
