Fixed and re-added client dialog for untrusted server certs

Updated georg/MemorizingTrustManager code to snapshot of 5e1c3a38a.
Uncommented MTM hooks in preferences XML and SSLConfig.
Updated AppRTCActivity to prevent activity from exiting when the
MTM dialog pops up (the onPause() method only exits the activity if
the connection is already in proxying mode).