API to set multiple CA certificates for WifiEnterpriseConfig

Support multiple CA certificates as trust anchors for server certs
when connecting to EAP networks. For the full feature, also requries
wpa_supplicant support, Wifi stack change to set up certificates and
aliases, and Wifi Setting UI tweak.

Bug: 22547958
Change-Id: I3c9f0e184632b79044a9e623ce0e7d9aaddd2ae3