Store ICE fallback permission in device setting

This stores the ICE server fallback permission in a device setting so it is
remembered across sessions.

Part of https://github.com/matrix-org/matrix-react-sdk/pull/3309

src/settings/Settings.js

@@ -372,4 +372,10 @@ export const SETTINGS = {
         default: false,
         controller: new LowBandwidthController(),
     },
+    "fallbackICEServerAllowed": {
+        supportedLevels: LEVELS_DEVICE_ONLY_SETTINGS,
+        displayName: _td("Allow fallback call assist server turn.matrix.org"),
+        // This is a tri-state value, where `null` means "prompt the user".
+        default: null,
+    },
 };