Playback: improve podcast detection, fix continuity and shuffle

This improves the podcast detection, lowers the threshold to 30 minutes to include some Opera sessions where progress is desired and correctly classifies other edges cases where it failed previously (notably audio with a nil genre).

Additionally, this ensure that the first played item in shuffle mode is actually random and not index 0.

Finally, it let's the playback service and not the UI handle the playmode state restoration to prevent conflicts.