Unhide WifiConfiguration proxy accessors

Unhiding HttpProxy accessors in WifiConfiguration.
This change occurs in parrallel with changes that restrict updating of
WifiConfigurations with new or updated proxy info to uids
holding permission OVERRIDE_WIFI_CONFIG, or holding DeviceOwner or
ProfileOwner policies.

Bug: 14669153
Test: None
Change-Id: Id69525cfec152ea52697693f9a3bd6f86019c694