Avoid rebuffering after RTSP pause

If pausing an RTSP stream, an RTSP Pause request is sent and then
if the stream is immediately resumed again, an RTSP Play request
will be sent to the server.
But the new data after the pause will not be buffered until
Sender Reports have arrived again on both channels.
Meanwhile the player will resume playback and continue consuming
the already existing buffer.
This means that there is a risk that the buffer is emptied while
waiting for sender reports.

This commit simply adds a delay before the RTSP pause request is
sent, allowing some additional RTSP buffering that might be needed
when the stream is resumed again.
Also, if the stream is resumed again before the RTSP pause request
is sent, there is no need for any RTSP pause request, hence it is
omitted.

Change-Id: I928c8bfb5e99a6a146dcda4e51e528973ecbe065