Fix NPE in AutofillManager when feature autofill is disabled

When feature_autofill is not present on device IAutoFillManager
service could be null. It is proposed to check service for null
before trying to add client to prevent NPE.

Bug: 180913556
Test: cts-tradefed run cts-dev -m CtsWidgetTestCases \
    -t android.widget.cts.DatePickerTest#testPartsVisibilityInSpinnerMode
Test: atest CtsAutoFillServiceTestCases
Change-Id: I4d65fe4f664e37115d653e981094f1a52c1d97e8