[Wi-Fi] Check if domain field is not empty when users choose a ca certificate

Domain is required for client to validate server's certificate.
This change disable Save button for a new Wi-Fi network of a
specified CA certificate but empty domain.

Bug: 161378819
Test: make RunSettingsRoboTests ROBOTEST_FILTER=WifiConfigControllerTest
      make RunSettingsRoboTests ROBOTEST_FILTER=WifiConfigController2Test
      1. Install a certificate.
      2. Add a new Wi-Fi network of EAP method PEAP
      3. Set CA certificate to the certificate installed at 1.
      4. Save button should be disabled.
      5. Input something in domain field.
      6. Save button should be enabled.
Change-Id: I65c8b29ec7a03b21403ddbfc087ce48c2a4a69e4
4 files changed