Call onServiceConnected only after callback is saved in the service.

Prevent the onServiceConnected to be called prematurely. Until now
it was called in the manager and so that a client may rely on the
callback being called before it was actually available to the service
to use.

Bug: 293511169
Test: atest android.net.wifi.sharedconnectivity.cts
Change-Id: I698378a057990bd8ea73974130fa29ab1cd7c738