Add "wifi lost" disconnect cause.

New disconnect cause used when wifi call is dropped due to loss of wifi
connection.

Test: Manual, unit.
Merged-In: Iedaa9f54ff6dbfe90a6ce01fc9eb0aad126d05f0
Bug: 32571209
Change-Id: Iedaa9f54ff6dbfe90a6ce01fc9eb0aad126d05f0