Added a new AutofillCallback.EVENT_INPUT_UNAVAILABLE event.

This event is called when:

- After notifyViewEntered() when autofill is disabled.
- After service returns an "empty" FillResponse to FillCallback.onSuccess().

BUG: 36056207

Test: LoginActivityTest.testAutofillCallbackDisabled
Test: LoginActivityTest.testAutofillCallbackNoDatasets
Test: VirtualContainerActivityTest.testAutofillCallbackDisabled
Test: VirtualContainerActivityTest.testAutofillCallbackNoDatasets

Change-Id: I7b8636473f738bf600aa96b28c77827b2cc78815