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.cpp

@@ -6,6 +6,7 @@
  */
 
 #include "ConnectionContextStore.h"
+#include <folly/Optional.h>
 #include <folly/json.h>
 #include <folly/portability/SysStat.h>
 #include <openssl/err.h>
@@ -110,6 +111,21 @@ std::string ConnectionContextStore::getDeviceId() {
   }
 }
 
+folly::Optional<FlipperCertificateExchangeMedium>
+ConnectionContextStore::getLastKnownMedium() {
+  try {
+    std::string config =
+        loadStringFromFile(absoluteFilePath(CONNECTION_CONFIG_FILE));
+    auto maybeMedium = folly::parseJson(config)["medium"];
+    return maybeMedium.isInt()
+        ? folly::Optional<FlipperCertificateExchangeMedium>{static_cast<
+              FlipperCertificateExchangeMedium>(maybeMedium.getInt())}
+        : folly::none;
+  } catch (std::exception&) {
+    return folly::none;
+  }
+}
+
 void ConnectionContextStore::storeConnectionConfig(folly::dynamic& config) {
   std::string json = folly::toJson(config);
   writeStringToFile(json, absoluteFilePath(CONNECTION_CONFIG_FILE));