WifiConfigController: set ca_path and ca_cert to null if unused

The WPA supplicant ca_cert and ca_path directives should not
both be non-null, since our EAP settings logic only allows one
or the other to be used.

Modify logic in getConfig() to explicitly set ca_path or ca_cert
to null if it is not used. This explicit null value is necessary
to override a previously non-null value saved in an existing
configuration.

Also, always set the domain_suffix_match directive, which
will lead to this directive to be reset when the Domain
field is hidden.

BUG: 27194668
TEST: 1) Install custom ca certificate onto device as "testcert"
TEST: 2) Configure an EAP-TLS network with CA certificate
         "testcert", Domain "testdomain",
         User certificate "Do not provide", identity empty, and save.
TEST: 3) Read network variables in data/misc/wifi/wpa_supplicant.conf
         and ensure that ca_cert is "testcert", ca_path is not
         present, and domain_suffix_match is "testdomain".
TEST: 4) Modify that same EAP-TLS network configured in step 2 with
         CA certificate "Use system certificates", Domain "testdomain",
         User certificate "Do not provide", identity empty, and save.
TEST: 5) Read network variables in data/misc/wifi/wpa_supplicant.eonf
         and ensure that ca_cert is "keystore://CACERT_testcert",
         ca_path is not present, and domain_suffix_match is
         "testdomain".
TEST: 6) Modify that same EAP-TLS network configured in step 2 to be
         an EAP-PWD network. Leave Identity and Password blank, and
         save.
TEST: 7) Read network variables in data/misc/wifi/wpa_supplicant.eonf
         and ensure that ca_cert, ca_path, and domain_suffix_match are
         not present.

Change-Id: I547f3e359bc8e9b77e51e10e60356b857230636f