WifiConfiguration: deprecate WEP-related fields

WEP provides poor access control, poor
confidentiality, and reduces throughput. (On the
throughput front, WEP connections are limited
to 54 Mbps, even on access points that support
transmitting at speeds 10x as fast.)

Mark WEP-related fields as deprecated, to
communicate our intent to deprecate WEP connections.

Bug: 72332080
Test: m -j8 update-api
Change-Id: I0366341a62711c5f4a2e7d1bbf049a5e058d0ec2