[Feature Request] Per-MediaItem default start position in playlists

[REQUIRED] Use case description

In a catch-up TV / continuous replay app, the player transitions to the next program when the current one ends. The next program often needs to start at a non-zero position (accurate start offset without pad-in for the overlapped tv shows).

We want to use PreloadConfiguration + player.addMediaItem() to preload the next item and get instant transitions. This works when starting at position 0, but there is no way to set a per-item start position that would be respected during auto-transitions. Timeline.Window.defaultPositionUs is always 0 for queued items.

Proposed solution

Add a defaultStartPositionMs to MediaItem that sets Timeline.Window.defaultPositionUs for that item:

val nextItem = MediaItem.Builder()
    .setUri(nextShowUri)
    .setDefaultStartPositionMs(bookmarkMs)
    .build()

player.addMediaItem(nextItem) // preloads from bookmarkMs, plays from bookmarkMs

Unlike ClippingConfiguration.startPositionMs, this would be non-destructive — the user can still seek before that position.

Alternatives considered

• ClippingConfiguration.setStartPositionMs() — Destructive: prevents seeking before the clip start.
• seekTo() in onMediaItemTransition — Causes audible/visible stutter; wastes preloaded data from position 0.