Prevent NPEs when registering/unregistering ConnDiags CBs.

This CL updates ConnectivityService to do null checks on received
parameters when registering and unregistering ConnectivityDiagnostics
callbacks (and also when simulating Data Stalls).

Bug: 181583568
Test: atest ConnectivityServiceTest ConnectivityDiagnosticsManagerTest
Change-Id: I7f297f10bf8d379a5d33ca5e11ca1e12132ba3a5
2 files changed