winrt: Depend on access to service for supportedPositioningMethods

The locator's status is only updated while a positioning session is
active so it does not always reflect the correct state. By checking the
access we have a more reliable way to reflect the current state.

Task-number: QTBUG-66427
Change-Id: I75d2274c9efd29bf070913246738bf18a9ec5edc
Reviewed-by: Miguel Costa <miguel.costa@qt.io>