Create TestApi for simulating a Data Stall on ConnectivityService.

This change adds a TestApi for simulating a Data Stall to
ConnectivityService. This allows for Data Stalls to be triggered without
having to manipulate the signals used by NetworkMonitor . This also
allows NetworkMonitor to update the ways it detects Data Stalls without
affecting CTS tests for ConnectivityDiagnosticsManager.

Bug: 148032944
Test: atest ConnectivityDiagnosticsManagerTest
Change-Id: Icad439efa2ab4c872c21d3ee6ceaae8c5b49f18d