Fix ZXing Wi-Fi QR code parsing bug.

ZXing Wi-Fi QR code uses ';' as the delimiter for key/value pairs,
should not treat an escaped "\;" as the delimiter.

This fix also change the parsing result:

  If there is no specified key, the result value is null.
  If specified key exists with empty value, the result value is an empty string.

Bug: 118797380
Test: atest WifiQrCodetest
Change-Id: I786ce7c4fa66dcb31d8a61d7a3251c2f539ccc99