Invoke onConnectivityReport on registering ConnectivityDiagnostics.

This change updates the behavior for registering
ConnectivityDiagnosticsCallbacks. Now, after a successful register()
call, callbacks will receive cached ConnectivityReports for all
matching, permissioned networks. This allows registrants to be updated
with the network state for their networks without having to wait for the
next network validation.

Bug: 147849853
Test: atest FrameworksNetTests
Change-Id: I924ba8fdcc847f453557021591bde38602fe089c