Add AutofillService#onSavedDatasetsInfoRequest.

System settings will use this to display the number of saved passwords
in each AutofillService available on the device.

The API supports some flexibility to make it easier to support a
different settings UX in the future.

Bug: 169455298
CTS-Coverage-Bug: 183499221
Test: manual - check the result displayed in settings
Change-Id: I13dd2f06dad18cfe8ef5de9b44854e44ac8d7365