When trying to restore a keyback that has its key in 4S, check if 4S will be readable

If the account data for the 4S key is cleared or lost somehow, don't attempt to
read the key and ask for the passphrase/recovery key instead, as doing so would trigger another
bootstrap from the restore key backup dialog, overwriting the existing key backup.

src/components/views/dialogs/keybackup/RestoreKeyBackupDialog.js

@@ -243,8 +243,10 @@ export default class RestoreKeyBackupDialog extends React.PureComponent {
             loadError: null,
         });
         try {
-            const backupInfo = await MatrixClientPeg.get().getKeyBackupVersion();
+            const cli = MatrixClientPeg.get();
-            const backupKeyStored = await MatrixClientPeg.get().isKeyBackupKeyStored();
+            const backupInfo = await cli.getKeyBackupVersion();
+            const has4S = await cli.hasSecretStorageKey();
+            const backupKeyStored = has4S && await cli.isKeyBackupKeyStored();
             this.setState({
                 backupInfo,
                 backupKeyStored,