Disable configuring EAP-TLS without user certificate.

Disallow configuring EAP-TLS without a user certificate,
since this is probably an invalid configuration.

Also:
- change the wording of the default option for
  EAP CA and user certificates from "(unspecified)" to
  "Please select", to make it obvious that a choice
  needs to be made to proceed.
- Fix style issues found in WifiConfigController.java by checkstyle.py.

BUG: 26686071
Change-Id: I7ccfdf40db97328e3297a03cc43033ff2428980f
TEST: Save option is grayed out when configuring EAP-TLS network
TEST: while leaving "User Certificate" left at "Please select".