ohos: drop withContext() in position info source

withContext() is used in makeSinglePositionInfoUpdateProducer() only to
keep the position update proxy reachable from both the then and the
catch callback of the getCurrentLocation() promise. Build the proxy
behind a shared_ptr right away, the way registerLocationErrorHandler()
and the satellite info source already do.

No functional change is intended.

Part of removing QNapi::PromiseWithContext.

Pick-to: 6.12
Task-number: QTBUG-150038
Change-Id: I3a590ac31cf791824f4cdf7045589015297d3292
Reviewed-by: SanthoshKumar Selvaraj <santhosh.kumar.selvaraj@qt.io>