Introduce ServiceConnection#onNullBinding()

Historically, if a service returns null from onBind(), the binding app
gets no information about the outcome:  the ServiceConnection is never
invoked.  We now introduce a new connection callback, onNullBinding(),
for apps that need to detect this situation.  When the service rejects
the binding by returning null, the onNullBinding() callback in the
associated ServiceConnection is invoked instead of onServiceConnected().

onNullBinding() has an empty default implementation, so there is no
binary-compatibility impact of this new interface method.

Bug: 67377345
Test: atest android.app.cts.ServiceTest

Change-Id: I224512c118f7d6e5c1c2bb69eca1902882e73594