Update javadocs for ConnectivityDiagnosticsManager.

ConnectivityDiagnosticsManager comments for
registerConnectivityDiagnosticsCallback and
unregisterConnectivityDiagnosticsCallback are updated to reflect several
changes.

For register calls, any app will be able to register callbacks, but only
permissioned applications will have their callbacks invoked (and only
for networks managed by the application). Additionally, only the
registering app (uid) will be able to unregister a callback once
registered.

Bug: 143187964
Test: docs change only. compiles.
Change-Id: Ie7ae86a1afccb22d6c84027dbac49d7b8e431e8c