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: 174444980
Test: cts-tradefed run cts-dev -m CtsWidgetTestCases \
    -t android.widget.cts.DatePickerTest#testPartsVisibilityInSpinnerMode
Signed-off-by: Artem Radchenko <artem.radchenko@globallogic.com>
Change-Id: Id6bad7e652452343ed674e4a7452a1b8231f72c5
Merged-In: I10ce988603f47117d0efddcc014ec51b72d3b3b4