[rbp] Fix elapsed time not updating near end of file

Currently, once demuxer has reached EOF, we send the EOF messages to audio/video players
and set their inited/started flags to false. But if started is false we ignore any
PLAYER_DISPLAYTIME coming back from players, which stops the elapsed time from updating.
This can affect the final ~8 seconds of the file.

The fix delays changing these flags until audio/video players have signalled EOS.