Implement INetworkMonitorCallbacks#notifyDataStallSuspected.

INetworkMonitorCallbacks defines notifyDataStallSuspected() for
notifying ConnectivityService of networks encountering a potential data
stall. A new event is introduced for ConnectivityDiagnosticsHandler to
process the notification and invoke the relevant
ConnectivityDiagnosticsCallbacks.

Bug: 143187964
Test: compiles
Test: atest CtsNetTestCases FrameworksNetTests
Change-Id: I70320bdda9855dced31e08e6a0b25329fb5cb535
(cherry picked from commit 6c51dc9de39d7963a6bc6d03eacadf2ba2131e20)