These have the same value (`-1`), and basically the same meaning (offset
in an array/list/file/byte stream/etc), but 'position' is an overloaded
term in a media playback library, and there's a risk people assume that
methods like `Player.getCurrentPosition()` may return
`C.POSITION_UNSET`, when in fact unset media times (whether duration or
position) are always represented by `C.TIME_UNSET` which is a) a `long`
(not `int`) and b) a different underlying value. (aside:
`getCurrentPosition()` never returns an unset value, but it's a good
example of the ambiguity of the word 'position' between 'byte offset'
and 'media timestamp'.)
PiperOrigin-RevId: 492493102
