RTP: Send silence packets on idle streams for every second.

Originally a stream does not send packets when it is receive-only or there is
nothing to mix. However, this causes some problems with certain firewalls and
proxies. A firewall might remove a port mapping when there is no outgoing
packet for a preiod of time, and a proxy might wait for incoming packets from
both sides before start forwarding. To solve these problems, we send out a
silence packet on the stream for every second. It should be good enough to
keep the stream alive with relatively low resources.

Bug: 3119690
Change-Id: Ib9c55e5dddfba28928bd9b376832b68bda24c0e4