SDK / exoplayer

DefaultDownloadIndex is currently just forwarding all calls to its
inner class. getDownloadTable() handles initialization, but this
doesn't really seem different to having an initialization method. It
doesn't guarantee initialization happens, since a bad method
implementation could try and access downloadTable directly, just as
a bad method implementation could forget to call the initialization
method. Hence this change removes the indirection.

PiperOrigin-RevId: 232482228