Fix content resume offset calculation for asset list interstitials

When an HLS interstitial uses an asset list and does not specify a resume offset, the content resume offset should be based on the total duration of the ads in the asset list. This change updates the calculation to correctly use the sum of the asset list durations when resolving the content resume offset.

Issue: androidx/media#3322
#cherrypick
PiperOrigin-RevId: 948926502
(cherry picked from commit 69ff047d896704e458882b6eb188e01596967de8)