Fix potential strict mode violation when stopping RTT.

When the RTT call was cleaned up, the "close()" method was not being
called.  In practice I think the Dialer app never really stops RTT, but
the APIs (and test app) do allow it to be stopped mid-session.  I'm
hypothesizing that is why we don't see this.

Test: Use Telecom test dialer to start and end RTT session.  Observe no
strict mode violation.
Fixes: 186869727

Change-Id: Id474b493ac3fe39e97360f3ac326279fc9a9a5eb