Run callbacks on ConnectivityService thread

Run MultinetworkPolicyTracker and DataConnectionStats callbacks on the
ConnectivityService handler thread.

Previously the callbacks would be using the SystemServer foreground
thread (Looper.myLooper()), or the broadcast thread for the
MultinetworkPolicyTracker BroadcastReceiver. This is error-prone, can
cause threading issues and makes it difficult to test the components.

Test: atest FrameworksNetTests
Change-Id: I189213dd363004abed294659165bf5430d153bba