Implement ConnectivityManager.reportBadNetwork() to trigger network validation.

Network traffic used to perform the network validation is billed to the UID of
the caller of reportBadNetwork.  This change does not change the actions taken
upon validation failing or succeeding:  NetworkMonitor will show the sign-in
notification if a captive portal is found.  NetworkMonitor will inform
ConnectivityService if a network tests functional.  NetworkMonitor will not
take action if a network lacks any connectivity.
Also, remove an unused Thread that was confusing bandwidth billing.

bug:17326268
Change-Id: I7fea23480af54211004a0a1c535a71c2793f21bb