Move Data Stall logic to CS from NetworkMonitorCallbacks.

This change moves the logic for handling Data Stall notifications from
NetworkMonitorCallbacks to ConnectivityService. This avoids duplicate
logic for managing data stall simulation requests from
ConnectivityManager. This also puts all of the logic for proxying Data
Stall notifications to the ConnectivityDiagnosticsHandler into one
place.

Bug: 148032944
Test: atest ConnectivityDiagnosticsManagerTest
Change-Id: Ie2f6a1a2376c5c452750ab417cb5e8c24fc44fc3