Clean up RTT streams properly after termination

After downgrading an RTT call to a voice call, we weren't cleaning up
the streams properly, which meant that new streams wouldn't be transmitted
to the connection service if we upgraded to RTT again.

Fixes: 153683581
Test: manual -- upgrade and downgrade RTT over and over again
Change-Id: I05eef906bedf223e9ef89711559a12806aa2822f