Wifi usability: Add Wifi usability score into Wifi.proto for metrics collection

Wifi usability score together with other related stats are added into Wifi.proto.

Bug: 113262380

Test: Unit tests for Wifi

Change-Id: I451eb9914f47d17eb42016b6d13f1f1759e17f48
Signed-off-by: Mingguang Xu <mingguangxu@google.com>