Initiate cert exchange when medium changes

Summary:
Changelog: Initiate cert exchange when medium changes

Fixes the following bug:

1. Start an iOS app with WWW cert exchange
2. Get cert (and fake serial of a virtual device)
3. Close app
4. Restart Flipper
5. Switch app to FS_ACCESS cert exchange
6. Start app

Expected result:
App re-initializes cert exchange to get a new serial of a real device

Current result:
App tries to connect securely using the previous fake serial of a virtual device. Flipper does not recognize it and refuses the connection.

Reviewed By: lblasa

Differential Revision: D33163798

fbshipit-source-id: 67126a9b562f2cb7cfe6521a46abf38b2699fb2d

xplat/Flipper/ConnectionContextStore.h

@@ -7,9 +7,11 @@
 
 #pragma once
 
+#include <folly/Optional.h>
 #include <folly/dynamic.h>
 #include <folly/io/async/SSLContext.h>
 #include <string>
+#include "FlipperCertificateExchangeMedium.h"
 #include "FlipperInitConfig.h"
 
 namespace facebook {
@@ -24,6 +26,10 @@ class ConnectionContextStore {
   std::string getCertificateDirectoryPath();
   std::string getCACertificatePath();
   std::string getDeviceId();
+  /**
+   * Get medium over which the certificate was received.
+   */
+  folly::Optional<FlipperCertificateExchangeMedium> getLastKnownMedium();
   void storeConnectionConfig(folly::dynamic& config);
   bool resetState();