gps: fixed singleshot session that doesn't stop

When a single shot location is requested and a
location is returned, the location request is
not stopped and so it continues even when the
requestor has stopped listening. This can cause
gps tracking to continue in the backgroud and eat
up battery.

This is fixed by stoping the provider if there
are no more update record associated with it
when a location is returned.

Change-Id: Ie437732ff06231a46724843af30e21ca3a1694ab