sip: force SRTP when TLS is enabled

SIP mandates a secure media transport when using
a secure SIP transport. To respect this, Ring
disables media when a secure call has no
secure media configured (sipcall.cpp:834).
Instead of failing, a better behavior would be to
force enabling media encryption when TLS is active at
the account level.

Change-Id: I25b77248be5d50bf016f959f0a06ec4850fa642e
Reviewed-by: Andreas Traczyk <andreas.traczyk@savoirfairelinux.com>

src/sip/sipaccount.h

@@ -377,6 +377,8 @@ class SIPAccount : public SIPAccountBase {
         }
 
         virtual sip_utils::KeyExchangeProtocol getSrtpKeyExchange() const override {
+            if (tlsEnable_ && srtpKeyExchange_ == sip_utils::KeyExchangeProtocol::NONE)
+                return sip_utils::KeyExchangeProtocol::SDES;
             return srtpKeyExchange_;
         }