Update to Diagnostic Callback flows for multilayer

Updates to ConnectivityDiagnosticsCallback flows that directly accessed
the NRI 'request' member to now instead use mRequests. This will be done
by first not allowing multilayer requests on registration of a
connectivity diagnostic and then accessing the '0' index of mRequests.

Bug: 174187714
Bug: 171991028
Test: atest FrameworksNetTests
atest FrameworksNetIntegrationTests
atest CtsNetTestCasesLatestSdk

Change-Id: Ida98036b70fa290a214ee23236bfd0aad08e90a3