Add functionality to set playlist item by ID

For cross-process use cases I am missing a way to seek to a playlist item by ID from the client.

Framework media has TransportControls.skipToQueueItem(long), where the parameter is a long ID. Media3 seems to have nothing that works across process boundaries.

There is Timeline.getIndexOfPeriod(Object), but it does not and cannot work across process boundaries because of the oddly unconstrained Object parameter type.

The only way I see to obtain functionality on par with framework media is implementing a custom SessionCommand myself. (I've simply made it constrain the Object parameter type to something usable and return the result of getIndexOfPeriod() to the client.)